I came across another interesting formula.

"Maybe some people don't know that it can sometimes be beneficial to divide the cows. The probability of an attack increases stepwise as the number of cows approaches a single unit.
For example, the 22nd cow has a 35% chance of killing 3 enemies.
But if you divide them into groups of 11, each group has a 30% chance of killing 2 enemies. This means there's a chance to kill 4 enemies at once, which wasn't possible with a single herd of cows!"