That's kinda what I was gonna say. I call it "where then engine is happy". In other words, high enough so it doesn't ever try to stall, yet low enough so you don't get any harshness shifting into reverse or drive from park. It boils down to personal preference, really. Although not really comparable, my carbureted slant 6 isn't really "happy" unless it;s pretty close to 1000 RPM. I've got it a tic over 900 now and so far it's not stalled. But it will every once in a while if I idle it down much lower.
