The return is tied to the overhang ... So it you put a HIDDEN and NOT EXTRACTED IN MODEL beam over the top of the wall and reference the roof to the beam instead of the wall, adjusting the beam location ad the overhang will adjust the location of the back of the return. Not ideal but it can work.
The whole birdbox return needs to be re-thought ,,, we need some control, if a porch roof is cantilevered, you end up with a massive birdbox the whole depth of the cantilever, and they are never built like that. Personally I'd prefer a ROOF OPTION - DIMENSION for the birdbox, then you can make them any size you want, and consistent throughout the project, or - even better - the same length control we have for the HIP RETURNS.