as BigBadDesignermon stated in this post:
dmg taken = atk - def
so, to know the % for def buffs we have to do this:
0) note your friend's def and add it to your total def
1) note dmg taken -> lets call it "dmg 1"
dmg 1 = atk - def
dmg 1 + def = atk
2) apply def buff and note dmg taken ( it have to be more than -1, try less def to do this) let call this "dmg 2"
dmg 2 = atk - def * X %
dmg 2 - atk = - def * X %
(dmg 2 - atk) / (- def) = X % <- this should be the % that gives the buff
3) lets collect those missing numbers! n_n
** def its total def of the team
for x-race buff a mono team of that race should be better, otherwise the math should be like this:
i.e.: supposing just 2 digis are affected by the buff
dmg 2 = atk - (def of those 2 digis) * X % - (def of the other 4)
[ dmg 2 - atk + (def of the other 4) ] / (- def of those digis) = X %
EDIT 1: there is a kind of problem, digis don't do a fixed dmg!
demon - from Trial 8 - 47 - 18 stam : did 996 one time, 1047 another time
554 and 566 with def buff on...
anyways with my def (2165, all dark members) and medium dark def+ those numbers translate into an increase between 19% and 22% -> can we call it 20%?
Medium increase in the defense power of purple dark allies = 20%+ def for darks
looking at the table that we already have
medium increase in atk power for x race also gives 20%+...
so looks like def buffs gives the same % as atk buffs...
small def - 5%
med def - 10%
high def - 15%
huge def - 20%
drastically def - 25%
small race def - 10%
med race def - 20%
high race def - 30%
huge race def - 40%
i'll keep testing to confirm this!