Here is how I made my building board flatter. This assumes I will be placing a new MDF surface on top of the existing surface.
1) 1st level the existing surface as best you can.
2) Using various thicknesses of aircraft ply, 1/32, 1/64th etc … I cut 1" squares of these
3) Then I lay out a grid on the table top say 10"
4) Using a long level place shims on the 2 to 3 low corners till I have level pads, fasten these shims down with CA as needed.
5) If your real picky you can even sand down the leveling shims to perfection, I use a real long sanding bar, the best part here is you only have to sand down the 1" squares.
6) Now I place 1/32" spaces on the corner pads and run a tight string across the corner shims.
7) Now I can place the additional shims at each of the points on the grid, I should have a 1/32 or smaller space between the new shims and the string. Glue down the shims as you go.
7) Again sand down the high spots as needed. When done I should have a set of perfectly level shim pads on a 10" grid.
8) Now I can place the new MDF surface on top and screw it down through the shims, Always place the screws so they go through a shim.
9) Then I place several layers (2) of dry wall as a pinning surface. As the surface gets messed up I just flip over or remove the piece of abused drywall.