Have you tried doing a round off, bh, rebound to your back on a resi at all? It really helps with the set. I think the process of measuring the tumbling distance has been therapeutic for some of my gymnasts. Once they can do it with ease it helps them see for themselves that they are consistent rather than just taking my word for it. We've played with the matstack too, adding some to see who can rebound the highest after their bh. It's a good visual drill, and gives them a tactile sense of their awesomeness that seems to be a good confidence builder for the no mat attempts.
Some of my gymnasts really wanted to train back tucks, but weren't picking up the difference in the set. I did round off + back tucks with them, and round off + bh's. In tandem with the mat training they eventually got it, and didn't have to sacrifice working on the skill itself.
