The Nikkel scans apparently used a saddle with no cut-outs for the stirrup straps. IIRC, they were called "Arizona bars". They save money in manufacturing, but the horse paid the price. The Nikkels will not make a saddle tree like that.
Seeing how much the rear moves when empty may be a decent 'poor boy's' method of checking for too much rock. If the saddle IS like a rocking chair on a floor, then the rear end will move more than one with better fit. But all will move some - unless they bridge. I was looking at trees on Mia's back and found one that was very stable. Then I looked closer, and I could slide my fingers under parts of the bar in the middle of the tree. It was very stable, and would have been very painful as well.
I honestly have a tough time figuring saddle fit on a finished saddle. I've tried sliding my hand under both with the horse stationary and with the horse moving, tried stretching legs, etc...but I'm pee-poor at checking fit past the first 6 inches of the front edge of a finished saddle. I've been wrestling with it on Bandit. He's slender, with a very straight back and a lot of shoulder. After a few months, it looks like I need to set the saddle further back than I do with my other horses, but the good news is that the saddle stays in place regardless of where I put it. Unlike Mia, Bandit IS a stoic horse, so I don't have a lot of feedback about something being "Owwy". He just deals with it.
I'm going to see if it is possible to rent a pressure pad, or hire someone to come out and do a check with one. I'd love
to see what is really happening with different saddles and riding styles. I'd also love to compare a good English & good western saddle with the same horse & rider. I used to think there would be a big difference, but now I'm not so sure. I'd also love to see how my Australian saddle would rate.
I also find the comments some folks have made on this thread about breast collars pretty interesting. I like their looks and people I respect say good things about them. It might be interesting to try. Yes, tack fascinates me.
I've got a couple of good back cinches, so I might pull one out of the closet and try it again. I did a couple of rides on Mia & Bandit with them - neither horse seemed to give a rat's patooty about it and I don't recall much difference...but I might try it again. There is a dirt road about 2 miles from here with some sharp drops - not long, but steep enough to get an idea. I'm dogmatic by nature, but old enough to know uncertainty has a wisdom of its own...