Kirby Allison does braces with an adjustable back section, to specifically alleviate this situation; the majority of button fix braces however only have front adjusters. Some clip on braces can also be adjusted at the back, which should resolve the issue.
Any use Curator? The centre image that TobyC's post above would seem to be the ideal position for the back joiner position to be; reasonably close to the shoulders; when the join comes lower down the back; that is usually where the problems occur; unless of course one has shoulders like Superman.