RE: But what about offhand...
For shooting offhand, I adjust my sling[s] so when I grip the forearm and then kinda tilt the bow so sling falls under my right elbow. When I mount the stock/butt to my left shoulder, the sling is not real tight but more than snug. I rest the stirrup on a bench I'm standing behind, take a few deep breaths and go from there. I don't know if it's my weak shoulder or just getting outta practise, but once in awhile I have to re rest the bow and take a few more deep breaths.
Been having a little fun w/ visitors my one friend brings by. He suggests to me we all shoot the xbows. The two I pulled this on so far didn't have a clue. What I do is tell them we are going to shoot a "William Tell" round. When they ask what that is, I get a tomato outta the bucket I picked and tell them they have to kneel at 30 yds w/ the tomato on their head while I shoot at it. It's a great joke, their eyes get as big as horse turds, lol. I've pulled this gag twice now and both times shot a perfect shot at 30 which then makes them wonder if I was serious, lol. Nothing like an audience to bring out the best in you!
Planter, I think concentration is key too. Sometimes I can keep quite a few in a snuff lid at 30, then there's the times when it's only 3 or 4 out of 5.