When I need to adjust an uneven tenon shoulder, I use a shoulder plane set for a thin cut. The trouble is, there isn’t much bearing surface for the plane’s sole to prevent me from accidentally rounding the shoulder’s edge. So here’s an easy fix: I clamp a thin board beside the shoulder and align it to the amount of material I want to trim off. It provides more contact surface for the plane to sit on and keeps the shoulder’s edge crisp.
– Alejandro Balbis