Partigyling an imperial stout recipe is rather easy. First batch is essentially a no-sparge batch. This leaves behind a good deal of leftover wort in the mash for the sparge to run into a second stout that will probably be sitting around 4.5% (unless it's a really big first running or you get 80+% eff). No-sparge nets me about 45-50% efficiency, and another 30% is very doable on the partigyle's second running stout. Don't do a mashout. It's helpful to have the sparge continue converting during the time your boiling the first runnings, and I only do 2 runnings. The second runnings is sparged.