Example: I print 8x8" on 11x14" paper. In a 16x20" overmat I mount it according to the attached picture.
The print is 8x8". The opening in the overmat is 9x9". There is 3.5" on each side of the opening, 4.5" on top of the opening, and 6.5" below.
To me it makes a very balanced frame.