I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.