some professor reroll arithmetic

base drop fraction = b
item drop bonus fraction = i

chance to get item = b*(1+i)
chance to get item with three rolls = 1 - ( 1 - b*(1+i) ) ^ 3
increased chance due to rerolls = 1 - ( 1 - b*(1+i) ) ^ 3 - b*(1+i)
equivalent amount as an item drop bonus = ( 1 - ( 1 - b*(1+i) ) ^ 3 - b*(1+i) ) / b