Assuming I have an HLT with the same size (or even a little bit smaller) than MLT, I just get it full of hot water, than while sparging it depends on what is my goal for that beer. Sometimes I just keep measuring the pre-boil volume that I want, in cases a couple gravity points below or above wont hurt and I want to achieve an specific color. Other times when I want to achieve an specific OG, I keep measuring the pre-boil gravity.
In any of these cases, how much hot water I have, and how much did I use in during the sparge? I don't know, because I don't need too. Once the boil kettle is getting close to the volume/gravity I want, I just stop adding water and let the rest of the wort I want get to the boil kettle.
Usually, just a few liters of hot water stay on the HLT (which I use for other purposes like washing stuff or re-hydrating yeast), and the rest of the wort that lasts in the MLT once you get used to this technique is really small volume. Basically.. doing that I never have to worry about sparge water volume... either for Fly or Batch Sparge (depends on the equipment I'm using).