The system is more or less like this:

Lets say damage is 50%

Healing 30%

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