I'll just quote a post I made in my town forum a while ago:
As you may or may not know, the developers have started a blog a few months back, revealing a few details of their workings and plans for the future. It has come to my attention that one of the developers, Zet, has posted the following in a comment in the blog:
Zet the Developer said:
Also you might want to know that the help does not reveal that the bonus is floating point which means that you can have an “uneven” bonus, for example 7.3.
This means that the assumption that any point in a Fort Battle skill is redundant if it does not increase your skill level to another bracket is false - the bracket being the ones described in the help file.
So the particular step-numbers (1,3,10,23,43...) don't mean anything! The fort bonus is just a
"regular" (probably continuous) function of skill points. A function with diminishing returns (so increasing your low skills is still the most important thing), but not a discontinous function as we had thought so far.
This means that boosting your Leadership from 43 (the minimum limit to get a +5 bonus) to 70 (just below the limit to get a +6 bonus) will benefit you! You'll not gain another +1 bonus (as you would by getting to 71), but you'd get a bonus of +27/28 ~ 0.96!
A good thing about this is that since the boni gained from skill points is no longer discontinuous, it's a lot easier to find an optimal set of items to wear for a fort battle.
This is why I now believe that
www.westcalc.info is actually correct (as opposed to just 'close to optimal'). If you use it, it'll show you the best gear (that you have in your inventory) to wear either when defending or attacking a Fort. It'll also show you your bonus as a floating (or decimal) number. I recommend it to all!
This post should contain 2 graphs. If you're having trouble loading them, just tell me and I'll see if I can fix it.
The following graph is how we all thought Fort Battle Bonus points worked:
As you can see it's a discountinous function that jumps up (the bonus increases by one) whenever the amount of skill points surpasses one of the step numbers (0, 1, 3, 10, 23, 43, 71, 108, 155, 211). What we used to conclude from the help file was that the above was true, and thus that increasing (for an example) Leadership from 50 to 60 had no effect. However, Zet has now revealed that this is not how it is to be interpreted.
A truer picture is shown in the continous graph below:
The truth is that inreasing Leadership from 50 to 60 does give an added bonus! However, it doesn't give an entire 'Bonus point' but only a partial one.
Exactly how much bonus it will provide is unknown, but since it's 10 Skill points in the interval from 43 to 71 (the two step numbers), it's probably a fairly good estimation to say that it gives a bonus proportional to the size of the step. That is to say that the bonus of increasing Leadership from 50 to 60 is approximately 10/(71-43) = 10/28 = 0.36 bonus point.