it counts like:
dmg*(bonus dmg)*(all %dmg from pets and other sources if stacks)

for example u have thing which says 2.00 dmg, your bonus dmg is 800% and u have pet with 25%dmg
so it will be 2*8*1,25 = 20 dmg

(bonus dmg depends only on your main stat, bonus dmg =100+mainstat/2)