DISCLAIMER: A lot of people have been involved in figuring out the DS formulas, so please do not think that this is solely my work.
DS DETERMINATION NOTES: There are a TON of factors involved in determining your DS. These formulas assume a perfect situation (or one where the variables can be identified and factored in). If you're bleeding from several locations, half health, loaded down with boxes, and bound by the creature.. your DS is not going to be anywhere even remotely close to what the formulas predict. However, your goal as a hunter should not be to predict your DS in those situations but to maintain the perfect situation instead.
STANCE/2 NOTES: In the parry DS formula is a bit about STANCE/2 (of which I'm not going to include (see the notes in evade DS for that reason)). The value for STANCE is 0 in offensive, 20 in advance, 40 in forward, 60 in neutral, 80 in guarded, and 100 in defensive. The result is added directly into DS with no modifications and is always present, except when attacked by a ranged weapon (at which point it becomes 0). However, there currently is a bug that if you're wielding a weapon you're NOT trained for, you still get that DS. For example, a shield using caster keeping the right hand open but with no brawling training gets this bonus against a ranged attack. If that caster had 1 rank of brawling, they would not get this bonus.
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On my 'to do' list is to make a spreadsheet for all this data. That one is going to be a while though (mostly because it's hard for me to make an aesthetically pleasing version).
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As of 6/30/09, all the formulas listed are functional to the point of being useable. The ranged DS formula and runestaff DS formula require a little more work to hammer out some kinks and the parry DS formula needs a little work with polearm DS, so there'll be more edits as time goes on (but there shouldn't be any major changes from what's already listed).
[EDIT (2/7/10): Just found out I left off a part of the SHIELD DS final calculation (dividing by 1.5)]
[EDIT (2/28/10): Added a minor note about TWC DS to the parry DS formula.]
[EDIT (5/31/11): I had stated, in the SHIELD DS formula, that you needed 1 rank to fully activate it. This is wrong and has been fixed in the post.]