It might be easier doing it that way, although I haven't tried it but thanks for the idea, I'll try that way. The issue is that you have to work "from the inside out" to the frame size, which is the rabbit where the glass and mat fits, not the outside edge - the size of the image on the paper, minus 1/8" all around so the mat will overlap the image by a little bit out to the rabbit of the frame. The mat has to hold the picture in place despite changes in temperature/humidity so that's the need for the overlap. The frames are much cheaper if bought in "standard sizes" which don't always come near to the 3:2 aspect ratio of the out-of-camera frame which means some cropping is needed. Sometimes that doesn't matter, sometimes it matters a lot.
Fractions are needed because that's the unit of measurement used by the mat cutter, which is extremely accurate to a 1/16th of an inch if you get the numbers right.
Then there's the aesthetics of the mat border. There's a simple formula I found in one of the books I read - the bigger the print, the bigger the border needs to be or the narrow mat border becomes a distraction to the artwork instead of complementing it. In short, bigger borders are better than narrow ones.
And the industry standard is that the rabbit of the frames are cut to 1/8 inch over the stated size for that expansion/contraction with temperature/humidity but some of the cheap Chinese ones are not so you have to measure carefully and compensate if using those.
And yeah, I'm getting "suicidal" over these damn fractions....
