So far I haven't bothered with mock-ups. I use tape to index the tube off the bender. Say the bend is 90 degrees, after the first one is made, I would place the bend next to a square, and read the distance from the tape to the far side of the bend. With that info, I can judge exactly where to insert the tube to complete the next 90 (or whatever) in order to get a given side length. It might be off 1/16" but that doesn't mater as long as the other side is the same.

Another thing that is important is to level (or whatever) the bender in a vise, then level the tubes (or angle then) for the next bends. That is all there is to ensuring the bends come out on the same plane. I guess that's obvious, but if you haven't tried it it may be less obvious that it works easily.

Getting the bends perfectly angled is probably the easiest part since you can nudge them to the perfect location.