Of course this is a personal taste thing but I am quite happy with white borders that are 6.8% of the long side (B+) of the image or 6% of the long side (B-) of the print (6% of long side image + 2 border widths). Would it be possible to add a percentage choice to border choices next to absolute width choices? Or more complex for framing, a number for the overlap in frame/matte + a percentage of the length of the image for the border width?  I guess it will be possible to make that overlap width with the second border setting as well but nevertheless.  Instead of the long side as a starting point the diagonal could be used too, computing is a bit more complex, percentage lower.

Met vriendelijke groet, Ernst

http://www.pigment-print.com/spectralplots/spectrumviz_1.htm
March 2017 update, 750+ inkjet media white spectral plots