thanks for all the replies.
it seems someone juust posted this: http://mmo-mechanics.com/swtor/forum....html#pid23668
which shows that kephess has a 90% accuracy melee attack, and that something like 68% of damage comes from these sort of attacks.
another 26% of the damage comes from attacks with 100% accuracy (and may not be affected by the accuracy debuff)
So 68% of the damage we effectivly get extra defense for our accuracy debuffs, but i am not sure about the 26%.
I think the way to proceed will be to have a two term model for mitigation that has two functions for defense.
where d1 is for 90% accuracy and acm is the accuracy matters term and acdm is the accuracy doesnt matter term, where d2 doesnt take accuracy debuffs into account