Mash in with your total brew water volume include all losses into this but drop brew house efficiency down around 60%.
I've full volume biab mashed one beer and didn't like the hit to brew house efficiency. Plus you have got to hear your wort to boil so you might as well sparge in that spare time.
Ah that makes sense. I'm been poking around at it with 5 gallon batches, it cuts about a half hour off for me as I don't have the equipment to heat the sparge water and begin the boil at the same time. Someday I'll have a killer rig, but right now it's a ghetto fabulous setup.
Same I heat up my sparge water on the stove. If you time it right you can reach sparge out temp and Finnish mash around the same time there is no dilly dallying then.