I've made about 200 knives and never got good at doing the bevels 100% on the belt grinder. My grinder runs too fast for good control. I grind for a while then straighten the bevel out by draw filing, then repeat as many times as necessary.
By draw filing I mean this; I use a single cut bastard file and grab it my both ends and use it like a draw knife running the length of the bevel, tip to handle and back and forth. I finish the bevel by wrapping sand paper around the file.
Hope this helps. Chad