I removed the fixed cup when I was overhauling a new-to-me mtb bike this fall and the lead mech mentioned they are called "fixed" because shouldn't be removed unless necessary - usually just clean and lube from the other side. Why are you removing it? (Just asking because I could learn something here...)

Also - be sure to use someting to hold your wrench onto the cup or it could slip off and bust knuckes or damage the fixed cup. I wonder if the reference to sheldon brown earlier was for that? Use large washers and a bolt going through to the other side of the frame to lightly clamp your wrench. Those fixed cups require a lot of torque to take off and put back on tight enough.