I use a sheet of drywall on top of my building table. Its easy to get perfectly flat. Put a straight edge from corner to corner & center to center & look for light under the straight edge. If you find a low spot, i use the newspaper or junk mail flyers to shim up the low spot in the drywall. I like it because, its cheap, stick pins stick in it, and masking tape readily holds down the plans and wax paper. After a few years, you can flip it over & start over w a fresh side.......Gene