Right now, my biggest paranoia is the flat surface. I believe it is probably critical. The bench I'm on is not perfect. I've laid a second sheet of ply to get the level portion done, but the flat part is bugging me. Likely, short of a complete bench rebuild, I will not fix it entirely. Another option in consideration....and please let me know your thoughts....is to lay a sheet of the 3/4 inch pink insulation board. It may not sit exactly perfect over any warp in the bench surface, but it should at the very least be FLAT.