I batch sparge, and regularly get 75%-80% efficiency, so I have never seen a reason to add another hour to my process by going to fly sparge, however if you are getting less water in the kettle then you want, you are not using enough in the first place. Try using .125gal per pound of grain for your grain loss compensation. So if I have 10lbs of grain, I know that grain is going to eat 1.25gals of water, So I need to add an additional 1.25gal to my batch sparge.
Batch sparge and fly sparge water are calculated the same way if your trying to meet a certain volume instead of gravity. If you want 8gal in the kettle with a 10lb grain mash, you will need 9.25gallons for your mash/sparge. Mash at what ever grain/water ratio you want, then fly sparge with the rest. If you end up with 8 gallons in the kettle, but gravity is low, then its more then likely you need more fly sparge water so lower your mash water, or your crush needs to be finer.
Either that or just mash/sparge and don't worry about volume, once you get your kettle volume at your desired gravity, even if the volume is higher then you want, who cares, no one says you have to add that extra liquid to your fermenter. If you end up with 6 gallons of wort at your desired 1.050, but your doing a 5gal beer, just add 5 gallons to the fermenter, that 5 gallons will still be 1.050
just decide what is more important, desired kettle volume, or desired kettle gravity. with batch sparge you don't really have to worry about either