You should take damae reduction in that formula somehow.<br/><br/><br/>an imp only does 1d4 damage, a pit fiend has DR 15 vs good and silver, nothing the imp's average of 2 can handle.<br/><br/>However with the formula, for example a (1000 imps / 20)*2= 100 damage, which would pass the DR<br/><br/><br/>So perhaps (# of attackers/20)*(average damage per round minimum - enemy DR)<br/><br/>