After you drain the first runnings from your mashtun, measure that amount of liquid. Take that from your final pre-boil amount, and then sparge with that much.
example- I drain about 2.4G of wort out in first runnings. I want to start with 6.5G. I need to sparge with another 4.1G water, either in one or two sparges. Either way, you're rinsing the grain, and it has already absorbed water, so you are likely to not lose anything due to deadspace or absorbtion while sparging.