Basically I eyeball it. I usually do a stop border, from 1/2 to 1", then I hold auditions for the final border by laying out the border fabric and just laying the quilt top on it and moving it around. I think the border width is also determined by the size of the quilt. If you have a queen size quilt with 4" blocks, your border can be larger than 4". And finally I really like to use large prints in borders so sometimes I base the border width on the size of the motif. Gee I guess I'm no help at all!