OP, I think “rail angle” means something even different, LOL.
I have a sausage with a narrow heart girth. She is an engineering nightmare to fit. Lots of saddles “stop fitting her” about where the stirrrup bars and waist of the panels are. (The waist of the saddle is the narrow bit of the hour glass shape they make.
IMO, they stop fitting her there because the angle of the rails is wrong, and then the panels follow suit but also, in so many saddles, are thick and broad there. In other words, the mare’s back changes from “normal horse” to round sausage very quickly in that section as you go from front to back.
So to me, she needs the rails of the tree to also get flatter, faster. If you were standing behind the naked tree and looked down it from the cantle, you’d see those steel rails being closer to horizontal than vertical just behind the stirrup bar.
I wish we have more vocabulary in English world. In Western world, the front-to-back curviness is called the “rock” of the tree.
Last but not least, the best-fitting saddles for this mare are a Tad Coffin Wide dressage (of course it is) and a Lovatt and Ricketts XW (hoop tree?) Berkeley. Both of these saddles fit around her rather than perched on top of her. She also has an Icon Flight with pony panels which I like and she says is “fine” in a “meh” kind of a way. She is a good sport about saddles, thank God, but when I put the Flight on her and see how far it sits above her back. IMO, pony panels make a real difference in how any Custom Saddlery saddle with a half-way right tree will fit the horse. I have both/all of these saddles because they have some form of a cut-back tree and put me father behind the withers than I want to ride. Sigh. It’s hard to find a forward-balance saddle that’s not French or Italian. And IMO, the Brits are best and building Sausage Trees. Sigh.
If you want to borrow my Lovatt and Ricketts to try, I’ll lend it to you. Or you might be able to get a demo from Adrienne Hendricks/Hendricks Saddlery in Eagle, ID, I have found her to be a good and honest saddler (she fits saddles as well as does legit, in depth repairs to them).