I think you won't find anything without a lot of assumptions and caveats; Any concave utility function will simply never lead to proportional giving.

But certainly, many people will scale up their own expectations of lifestyle along with their income until a pretty high point: that is, they'll try to tell you or at least behave as if they don't have a concave utility function, until they're well into the six figures (sometimes more). Since income changes are slow and tend to co-occur with many other life changes, this, along with psychological biases, complicate any earnest attempt to sit down and say "I'll be best off giving X portion of my income this year".