Like many amateur photographers, I make my own mounts for prints that I enter into competitions. I have some FrameCo Mat Master equipment.
I usually calculate the sizes for marking out and cutting both the piece of board and the aperture.
I have now made a simple spread sheet where the key dimensions are entered and the remainder are calculated.
The attachment below is a screen shot from the spread sheet (Excel). There is an alternative work sheet that calculates the overall size when the mount border widths are specified.
If anyone would like a copy of the spread sheet, PM me here with your e-mail address and I'll send it.
Terry

PS. The example shown is metric but you can enter in any units you like. 

Jeff,
I found an error this morning 
When corrected and checked, I'll send it.
terry

Terry, I'd like a copy of the spreadsheet.  I've got one coming up that will have multiple mats, can the spreadsheet work on those as well?
Tnx,
Chuck Tankersley