I go by the size I want and the fabric on which the border will touch, then how much more I want to add. I start off with a 1 1/2 inch or 2 inch, then go on to 6 inches , 4, or 5, again depending on how much more I want/need to add. I measure quilt top 1st , then depending on how much more I need, I will decide on the width and how many of the borders. Never have been one to follow rules, mostly I do what I find is appealing to the eye.