During legendary raids I find I usually land with 1 ball bonus for solo damage. I’m a level 35 using Go hub recommended type bonus pokemon, while my wife is a level 32 using game recommended pokemon. At the end of a raid we will both usually get 1 ball bonus for solo damage. In some cases I’m still on my 2nd mon while she could be on her 4th.
Raid groups vary in size from 7-18 players and still usually get one 1 ball. The only times I get 2 is when there is a group of 6-7 that are all levels below me.
Trying to figure out how this bonus stat is calculated?
Probably based on damage you dealt overall, I guess it doesnt take into consideration how many people were in the raid (so if you beat TTar in 4 you will get a bigger bonus than if you would beat him in 11 people)
It depends on damage dealt overall.
Back in the entei days we loaded up a single car and tackled the raid with just 5 accts. We could consistently get 14 balls each (3 damage bonus balls). So roughly 20% damage gets the max +3 balls 
.
Also +2 is roughly 10%, so around 10 players. (Assuming everyone does equal damage).