I try to figure the outside border at roughly 2/3 of the block size but never related to the narrow stop border except to make sure it is at least 3x as wide and maybe 5x as wide. But overall the outside border, I find, looks better if it is proportional to the whole quilt - in other words my 82" quilt would take a wider border, even if it's got the same block size, as a 56" quilt.
Of course this all depends on what fabric I've got left and how much of it!
