RE: How to get a perfectly flat (=plan, leveled) workbench?
MinnFlyer's suggestion is a very good and not an expensive choice. Generally $25 or so at Home Depot or Lowe's. I recently replaced our kitchen countertops and saved two formica covered pieces(one about 15" X 96" for a wing or fuse sides and one 26" X 48"). Both are still perfectly flat. Saving them for the next project. I and others have used tiles for drop down(hanging) ceilings on top of the solid surface. These make using your pins easier.