I bought two Certiflat 4'x3' tops without the leg kits, since I meant to make my own stand for them. I'm gonna butt them up side by side to have a 4'x6'. If you add their products to the shopping cart and leave them there a couple days, their system automatically sends you a 10% discount code , so I ended up saving the $99 shipping charge.
They say ±0.015" flat over 12", but it is easily flatter than that. The worst offender is along the diagonals. I got maybe an
absolute deviation of 0.042" using a 48" straight edge that is straight to ±0.003" IIRC, so that sorta translates into ±0.021" + 0.009" of possible (unknown) deviation of the straight edge=±0.030" total over 5 feet, so on average about ±0.006" per 12". I'd say that's pretty flat. I still need to finish the grand stand that I envision in my head (swiveling/rotating so as to have the top go vertical and save on floor space). Another back-burner project.