If you miter the corners, be aware that you need a *lot* of extra fabric -- basically the width of your 4 borders pieced together times 2 (one for each side) plus at least a couple of extra inches for safety. (Oh, and remember for the last two it's *4* times the pieced border because your second border measurements have to include the length added by the first two borders.)
A lot of people find mitering to be difficult. Because the sewn edges are all bias, there is a tendency to stretch them out of shape while you work with them. Once you get some experience with them, mitered corners are not a problem, but if you are already having issues with the measuring and sewing of borders, you might find mitering very frustrating.
You might want to focus instead on eliminating the problems you have with measuring and cutting the borders. Probably the easiest I have seen for cutting is to lay the border fabric along the middle of the quilt (quilt can be folded at the middle or not), cut a border to exactly match that middle, then cut the other border to exactly match the first cut. Fold both borders and the quilt in half and iron in a crease (or place a pin) at the halfway mark. Fold again to find the quarter marks. Pin ends and creases together before sewing, including extra pins between your crease marks. Iron the seams of the first two borders flat, then go through the same process again with the quilt for the remaining two borders. On the one hand it's a few extra steps to do this for four borders; however, when you sew all the borders together first and then have to miter, that last step of mitering can cause you to pull out your hair. (Do not ask me how I know this. I will admit to mitering multiple borders exactly once.)
Last edited by Prism99; 11-25-2016 at 05:32 PM.