New keggle

I'm using my new keggle tonight to do a 5 gallon partial mash batch. It's my 4th batch do bear with me. I am planning on mashing on my stovetop because I think I can maintain the 156* temp better. I will use 2 gallons to mash.

My main concern is how much water to have warming in the keggle during the mash. I did a dry run last night with water and noticed when draining I had maybe 12-18 ozs left in the keggle. I could always add water to the fermenter but I'm trying to follow as many of you guys techniques as possible!

Many thanks in advance!

I boil off 1.5 gal an hour in my keggel using a turkey fryer burner. So for a 60 min boil I start with Gal to end up with 5.5 gal post boil, lose .25 of crap on bottom of keggel and 5.25 to fermenter leaving me with 5 gal of great beer in the end.