Do you have a t-square? or anything that' make a 90 deg angle?
If so, lay it flat on its back, trim off the barbed edge with a razor knife. align an inside edge of the T-Square with the edge of the molding, move the t-square so the perpendicular piece is exactly where you want it, and run the razor knife along the perpendicular edge of the t-square.
No eyeballing, perfectly straight cut.