Calculating total runoff for partial mash?

I'm trying to figure out the maximum amount of grain I could partial mash, given the size of my equipment--the kettle is the main restraint (with my current kettle, about 4.25 gallon is the maximum practical boil size). I have coolers of various sizes I can rig up as a mash/lauter tun, so that's less of an issue at this point.

I know that strike water averages about 1.5 quarts per pound of grain mashed, and that it generally takes about 1.5 times the amount of strike water to sparge.

So that's approx. 3.75 qts of water per pound of grain that goes into the MLT at some point. I know that not all of the liquid actually makes it into the kettle. Is there any semi-reliable way to calculate the average amount collected from the sparge given the amounts of grain and water in the mash + sparge?

Thanks.

when i do big partial mashes i use 1 qt per lb grain, for initial strike @ 145, then add half that amount of boiling water for second step @ 151. i usually do 6-7 lbs. max of grain. I limit myself to 2 gallon sparge so after all is said and done i end up with about 3.5 gallons wort.

so for 6 lbs. grain i use 9 qt for the mash, then another 8 qt. for the sparge



so i guess i lose about .75 gallon.