To come up with the border widths for my quilt top, I MEASURED like crazy to make sure:
1) There is enough material of the color I'm using for the border
2) There is enough width for the border I'm attaching - I made sure to calculate the raw width x 4 plus one more strip to handle seams or use 4 blocks to handle corners.
So, start with your corners and determine how they should look (mitered, straight edge, rounded), then measure the length on each side and the width on the top and bottom. Now you can create a border strip that is that length plus seams for each piece a bit more. Then, lay the border out next to your quilt and it should come together.
Do a search of borders at the top of the page to get the exact details of how to measure to add on borders.