I always have a hard time accurately measuring marking cuts for mitered corners for boxes / frames to come out right so I made this jig and it works great.  The one end indexes off a 45 deg cut edge and provides a projected edge representing the corner will be and gives a hard edge for a tape measure.  you can then position the other block the exact distance you want for the inner length and mark the cut.  If you cut just to the outside of your line, it should come out dead on, I can get them within a 1/32 now.  For example, in the first image, this is is set up for a 11 5/16" as inner dimension.

After using I realized it useful to have the marking side reversible, so that change is in the model.