Two options for checking your shoulder setback on sized brass:
Wilson cartridge gage
Hornady Lock-N-Load headspace gage
If you use the Wilson gage, make sure there are no burrs on the rim of the brass before you check them. I drop them in the gage head first to check for burrs, before checking the shoulder datum.
Make small adjustments. The following picture via UncleNick shows how little movement changes shoulder setback. For semi autos, I goal for about 0.003" setback.