Take enough off so that the receiver is bare aluminum all the way around... You want to use a fair amount of pressure, but not for too long. Better to take it off early, clean and inspect, and take another pass or two or three than go for too long on one pass. Clean the lapping tool with each pass and use fresh lapping compound and oil on the extension that goes into the receiver. Mobil synthetic one is a good option to lube the extension of the tool.
Ya know what? I could just shoot it and see how it groups. It might be good to go right outta the box.