There isn't a magic number for the number of level your boosters can be over the boostee. Its just a matter of whether the mob is grey to the boosters or not. At 60, that number is around 9. At 70, that number is 7 or 8. I forget which. Regardless, if the mob is grey to the powerlever, you only get xp based off the % of damage you did. If the mob is green or higher, you get full xp.