MatboardCalc will give you the dimensions of your mat, board and frame, as well as where to cut the "window" in your mat board. Try different combinations of image dimensions, border sizes, mat window openings and board/frame sizes and MatboardCalc will update the results automatically.  

Microsoft Excel (or possibly similar software) is required. You can enter measurements in inches with fractions or decimals, or both, or use metric measurement.  

It's only 25 kb and downloads in seconds.