well i think it would work out like this.
count the number of models in the assault.
if you outnumber the enemy by 25% then 1-3 your men are hit if by 50% 1-4 your men are hit. if by 75%(which you shouldnt be shooting into by the way) 1-5.
also i believe the person shooting into combat should have to roll a leadership test to fire into his own men (you evil man)
if you are outnumbered by 25% 3-6 the enemy is hit, if by 50% and up 2-6
this i believe can accuratly depict the chaos that can occur when shooting into combat. it also helps display the fact that if you outnumber the enemy more accidents can happen. if the enemy outnumbers you your men have better chance of hitting the enemy.
i believe you should roll to see who is hit. then roll for bs and continue on as normal.