Replying to Backflips

So I can do backflips pretty well on a trampoline and into the pool and did some backflips on my skis into the airbag. The jump to the bag was a realistic one also (the guy who built it didn't know what he was doing so he didn't make it super poppy) which should help me. My question is though is that I still need to fix some things in the backflip, like waiting on the take off and popping a tad more straight rather than straight back and I need to get my head back. So should I build a jump in some soft snow and try it on that until I feel comfortable or should I try it on a park jump?.