Bill Tindall, E.Tn.

If I determined I could live with a not flat result I would knock off the diagonal corners on one side with a scrub plane (hand too content!) as much as I could stand and still have the thickness needed. Then I would run it through my thickness planer. And if it was a panel for a case side or table top, or even a panel for a door, I might not entirely clean up one side in order to preserve thickness.