If you want a mitered corner: add the finished width of all the borders, multiply by 2, and add 1", then add this to your regular border width.
In your example, the total finished width would be 7". You will need this on both ends, so multiply by 2 and add 1". 7 x 2 =15 + 1 = 15. If your quilt is 50 x 60, add 15 to each dimension and cut your borders 65 x 75.
I think this would be the length of your final border if you are doing it non-mitered.