There used to be a program called Matworks from a French author that was great, but I have not been able to get it to work in Windows 10
I remember that program but did not like it that much so I designed my own Excel spread sheet to do calculate the size of a mount. I use it a lot for preparing mounted prints for competition.
See attached screen shot of one example. The yellow cells are where dimensions are entered; the green ones are the calculated values.
Terry