I've got the 20 amp as well, and have tried two different approaches (this is with the Behmor). One is to set the Variac initially to keep it at a stable 120-122V, and then not touch it again. The other is to do the same initially, then begin to ride it again before first crack, say to 125V, then back it off to 117V almost immediately and try to space out/delay second. Then ramp it again to 122V before hitting cool.
The second approach has given me good results w/ P2...but then again, I really don't know what I'm doing.
The idea is to "help" P2 a bit.
Most importantly though, if I don't use the Variac, I frequently have initial line level at 115V or even lower and cannot get a decent roast.