I've always used the base of the bead/rib surface for the front and the rib surface at the reciever.

I've seen some ribs below the barrels at the muzzles, so laying it on a table would not give an accurate account of that type.

Also, I have checked my dinner table and breakfast nook table for flatness. Only certain parts of them are acceptably flat for measuring a gun for drop. It's amazing how curved some table surfaces are. Frankly, I like using a straightedge to get a good reading.