This is my stock answer to this question. It will depend on your brewing system. Hope this helps.
I like to start at the end and work my way to the beginning to figure water volume requirements. On my system I want 5 gal in a keg and half a gal below the fermenter spigot, to blowoff, and to yeast/trüb are wasted so I ferment 5.5 gal. 1 gal in my boil kettle below the spigot, to hop absorption, and to trüb is also wasted so I want my post boil volume to be 6.5 gal. I boil off a gal so I want 7.5 gal pre boil.
The water absorbed by the grain will vary with the specifics of the grain bill, the type of malt and adjuncts and their moisture content, but an average value of 0.50 quarts per pound (1.04 L/kg) has proven to be a very reasonable assumption in most cases. (Ref: BYO)
For 12# grain, grain absorption is ~1.5 gal and a bit in my pump and hoses is waste. My MLT has very little dead space. So, I start with a total volume of 9 gallons of water.
I like to try to even out my 1st and 2nd run (batch sparge) so I take the 7.5 gal I want to end up with in the brew kettle (pre boil) and split it in half. So my sparge will be ~3.75 gal.
9 gal total volume required - the 3.75 gal sparge (half the pre boil volume) = the rest of the total volume as strike water (5.25 gal). The 5.25 gal strike - the 1.5 gal grain absorption = 3.75 gal (equal runs).
Sent from my iPad using Tapatalk