short of buying a specific measuring tool I have laid SxS on table upside down and recently bought a long aluminum straight edge drywall hangers use. I am not sure if it is my eyes but I end up with different results.
Most SxS guns w/ an English style game rib will have the rib pitched so that the rib is slightly below the top line of the barrels @ the muzzle & if you use a straight edge over the rib to measure drop @ comb, face & heel the measured drop will be slightly different than if measured from the top of the barrels by laying the gun upside down on a table.
As Gr8day mentioned, having the front bead touch the table or straight edge will change the measurement also.