6" groups at 20yrds isn' t bad for a beginner, one thing I might make sure of is that you' re shooting a release? I was a 8" shooter at 20yrds pulling fingers, I bought a release and shot 12 groups at 20yrds under 4" with five arrows, scoring 24x' s. It was absolutely amazing the difference it made.

I' m also going to suggest carbon arrows if you' re not already shooting them, there' s jut no comparison in accuracy/trajectory, and you never have a straight alluminum arrow after they' ve been shot once, a carbon is either straight or broken, and they' re a LOT tougher, put an alluminum into a tree trunk and it' s a new arrow, put a carbon into one, assuming you can get it out, it' s fine.

It' s best to practice from the height you' ll be hunting at, but the difference isn' t much really, from 8ft to 15ft, you might not notice the difference (since you' re already a 6" group, if you were a nock-breaker, it would be fairly different, but you' ll be fine). A lot of guys never practice from an elevation at all, so the fact that you are is pretty impressive for a beginner.

How often do you shoot? Archery isn' t like rifle hunting, any hunter with a good gun can put 20 rnds a year through his gun and never be inaccurate enough to cause a problem out to 200yrds, but if an archer practiced like this, he' d never get accurate enough to hunt at 20yrds...it' s good practice to shoot twice a month in the summer, and once or twice a week in the early fall before season, then shooting during season is also important. Practice practice practice. You' ll cut your groups down in a hurry if you shoot that often.