Marguax the edges of a tree bar are beveled off or rounded off a hair they are not square as your theory presumes and if your padding does not conform to the shape of your horses back you need to re-examine what type of padding you are using. Also in your picture after printing it out and doing some cutting and comparission its funny how the shape used in the top drawing does not match the one on the bottom, if you are not comparing apples to apples it is not a accurate comparisson.
We run into this same type of thing with people who seem to think that in order to get the fork and horn sitting closer to the horses back that the gullet needs to be widened to allow everything to sit lower when in reality what they need to do is have the fork made lower in the gullet to achieve this, with the primary gullet height measurement being the back at the handhole not the front lip. This whole thing can get quite complicated and for years we have been looking at it in the wrong way due to articles and information that has been passed down that was flawed to begin with. Greg