For a print of that size, an inch more in matting at the bottom than at the top (in addition to the extra signature space) is too much for me.
I do what Dan does, sort of. 3/8" sides and top between print and mat, and 5/8" at bottom for signature. I find that bottom-weighting the mat by 1/8" is sufficient. So, doing the math, the print is 3/8" above center in the frame. I think an inch more at the bottom would be a distraction.