Here's the plan for the math. I'll introduce it and then I'll explain what it all means in a bit more depth. You'll have to click to enlarge and might want to put it in a new window to be able to reference and read simultaneously.
The game spans 15 levels and is roughly broken into three tiers of 5 levels each. I am not planning a big shift in the game at each tier like 4e; it is one fluid progression. The tiers are used for when things are able to increase (for instance, an option to customize a monster adds +2 damage at 1-4 and +5 damage at 5th level and higher).
The first set of columns are core numbers that are relevant to pretty much everything. They are the level bonus (1/2 level) and your stocks. Stocks are received at every odd class (as opposed to character) level and, for the most part, improve attack and defenses by +1. Stocks also have a rider power, like granting a handful of hit points, increasing damage, increasing wound threshold, and so on. There is a bit more nuance to it all, but that is the framework.
The importance of the "primary stock" versus "secondary stock" is the order in which they are taken. Because you only get a single stock at first level, and a stock improves attack or defense, you have to decide which one to improve. The one you improve first is the primary. Looking back at the "secondary stock" column, we then see that the table assumes you improved the secondary stock at 3rd level (when you'd get your next stock if you are a single class character). This is illustrated more clearly in the table below. The titles "primary" and "secondary" have no meaning in the game--you don't *really* even call them anything different. They're all just stocks. The reason I include it here is for clarity and completeness.
The remainder of the table bases all math on the premise that you are using your primary stock. This means that at various points (about half the levels--look at levels 1, 2, 5, 6, etc to see what I mean) the math assumes you are +1 better than you actually are at about half the things you do. In light of self-selection in actions, this doesn't have a big impact (not to mention that the game can stand up to a +1/-1 variance with ease).
I want to dwell on stocks a moment longer and talk about the different ways a player can customize a character through the stock selection. The stocks a class provides build on each other and are designed to be a progression. They also have higher requirements. For example, the Rogue has the following progression of attack stocks:
Deft Strike 1 (Attack 1)
Gain a +1 bonus to attacks with light or agile weapons. Gain a +2 bonus to initiative.
Deft Strike 2 (Attack 5)
Requirement: Deft Strike 1 or 11th character level
Gain a +1 bonus to attacks with light or agile weapons. Gain a +2 bonus to move checks.
Deft Strike 3 (Attack 9)
Requirement: Deft Strike 2
Gain a +1 bonus to attacks with light or agile weapons. Gain a +2 bonus to initiative and move checks.
Let's focus on the parenthetical. The word "Attack" just means that this stock is an attack stock. You can only ever have 5 of any one type of stock. The number is the minimum Rogue score needed to be able to take the stock. In this way, you cannot dedicate all of your stocks to, say, attack because you aren't a high enough level. Finally, the stocks generally improve, so you are encouraged to complete your track for the payout of the higher stocks.
Here are some sample progressions for how a character could pursue stocks:
The first progression is the assumption of the game. You pick one stock as your primary and alternate back and forth. The second progression just shows that you wouldn't *have* to do it that way and you could jump back and forth if you decide that you want to improve defense twice in a row or whatever. The third progression shows an ambitious approach involving multiclassing. This character aggressively multiclasses to get more stocks (from 1st class level) and puts them all into attack. This gives him an early boost in attacking but will cost him later on in that he won't be able to take as many of the higher level stocks. Long term, the character is not much better off numerically and continues to miss out on the riders of the higher level stocks.
There are a number of additional limitations of this aggressive approach. Many stocks have a built in limitation (such as the rogue attack bonus only applying to light weapons). If you take too many stocks from too many classes, you'll end up limiting your overall versatility. An even larger limitation is that you are aggressively taking stocks to improve numerically, but by not developing any one class you limit the potential of your "edges." To discuss edges, we return to the table up top.
Edges are the second set of columns under "Attacks." The concept here is that we develop a base number for your attack (ability + stocks) and then, usually, add a single number to it. By only adding a single edge to an attack, we can cut down on the problematic synergies that arose when bard song + charge + flank + higher ground + etc turned into an auto hit. If you only apply a single edge, there is always a single meaningful tradeoff that you are balancing. Quick and easy.
The first column titled "Edge" is the expected value when you activate a power that helps only you. For example, "Careful Aim: Action Point; add your rogue modifier to your next attack roll as an edge." You can spend an AP to have a good chance to hit. The second column titled "Group Edge" is the value of abilities that characters can activate and other party members can piggy-back on without personally having to spend an AP. A classic example would be bard song, but many classes have similar powers. Sometimes you can just freely add the edge (note that it still fills the one-edge per attack rule) and other times you have to do something to add it like use a maneuver (move action) to move adjacent to the ally.
You ability + stock + edge basically equals your attack roll. It is possible for things to add subsequently, but they are fairly rare. More often a character will choose to forego an edge that increases attack for some other benefit, like an increase in damage or the ability to ignore DR. In this manner, the mechanical constructs of stuff like Power Attack are actually embedded into the math. Instead of subtracting from your to-hit, you just don't add anything. It saves a step and better keeps things in line.
The next set of columns titled Defense just shows the expected progression of magic item bonuses to defense or the inherent bonuses people will give out to keep things balanced. Similarly, the "Totals" set of columns just sums Level Bonus + Primary Stock + whatever the header of the column says.
The next set of assumptions comes under the "Attributes" section. The game presumes an average stat modifier of +2 across all defenses. It is unlikely that characters will actually have this (at least to begin). So then why assume it? Because of self-selection and the power of pooling resources. Players will find ways to minimize the importance of certain defenses to their character, rules will be developed that allow you to replace Strength with Cunning for Fortitude or whatever. What is expected is that the average of defenses *important to the character* will be +2.
The assumption on attack stat is similar but embeds the idea that some weapons are +1 proficiency weapons (equivalent to the +3 weapons in 4e). Characters will either have a +2 with a +1 weapon or a +3 attribute. Granted, some characters will have a +2 attribute with a +0 weapon and other will have a +3 attribute with a +1 weapon. This is an average.
Finally we get to the "resulting scores" that show the expected bonus to attack and defenses. The attack bonus is calculated on the "Group Edge" total and it is the group edge that is the presumed default attack mode. The impact of the resulting scores is shown in the "Hit rates by attack mode" set of columns. The darker green shows, again, the group edge hit rates which are the default assumption of the game. We see that it is 60-65% across all levels. If you expend resources to use a personal edge, that number ticks up to as high as 75%. In the alternative, if you use your edge for something other than a bonus to attack, your hit rate steadily declines. The benefits in lieu of attack are balanced accordingly and there are many ways to try and improve this through teamwork or exceptional powers.
As a final note just to tie some disparate ideas together--I recently talked about "a provoke" providing a generic benefit against the provoking creature. That benefit is that you can apply a second edge to the attack by using a provoke. Immediately, we see the potentially awesome scenario of using a personal edge to have a high hit chance and then a provoke to increase damage.