Another solution from the program MatWorks! from Giorgio Trucco, that I use to calculate my mat cuttings:
Here you can define a larger bottom border for the matting style with the options "Bottom Weighted" and "Double Bottom Weighted". I think the larger bottom border is calculated with a percent value. Would that be an option for the border setup in Qimage?
Link to MatWorks! for all do-it-yourself mat cutters:
http://www.giorgiotrucco.com/matworks.html