The groupings you mention aren' t bad, but they' re not good either - at twenty yards I' m looking at hitting dimes.
A good source for teaching is the NFAA. From their site you can access/purchase videos by Bernie (i forget his last name). He is a very good teacher. I have improved my shooting markedly from watching a couple of his tapes.
He has a tape for learning yardage estimation, which is one that I would give a thumbs down to.
The others are quite helpful.