I bought a level with a laser in the end of it from Harbor Freight Tools for about $15. I sat it down on my table at one end with the beam shining down the table. I then used my six inch cabinet square as a target for the beam. I could easily read the levelness of the table to 1/32 of an inch (0.031). I then brought the top of the 1 inch thick plywood top to perfectly flat with a belt sander. It was messy and laborious, but it makes for an excellent reference plane. The two coatings of deck enamel have survived well during the construction of two planes.
I also found that mounting the laser on a camera tripod and using the line-making lens gave me an excellent reference for a straight line around the compound curve for the part line on my cowling. Jim Vance Claflin, Kansas va...@hbcomm.net