The system is more or less like this:
Lets say damage is 50%
Damage you block from Boss 20%
So the probability will Split between every hitter And healer. Then you Will multiply that % per the % of loot statistic of the rare or boss loot.
So the algorithm Will be something like this:
X = % of loot , D=damage u deal , H = healing people , DB= damage blocked from Boss, L=% item drop
X= (D+DB+H)* L