Thank you John.

I just had a look at 15x15 print in a 24x28 frame, it looks very nice. The 4.5 inch side borders are much better than 2.5 (which make the print look squeezed in). Also, at this frame dimension optically centered does look better than equal top and sides. The 24x28 frame is over 8 feet of frame though! The 22 x 24 looks OK as well. I think I'll buy the Neilsen frames wholesale and cut my own this time.....

Thanks for your help with this.