I'm having a hard time visualizing this method. What flat part on the bottom of the scope are you referring to?
Usually under the adjustment section it is flat. It is on my Vortex pieces. Get the optic tight enough that it takes a little effort to turn. Fill that space between the optic and the rail with the gauges. You will have to mix them up to find the exact thickness to fill the gap and hold the optic square to the rail.