So many things to take into consideration....how much fabric is available for the border, do I want to buy more? (usually!), how big do I want the quilt to be, how big is the batting I have available.
Lots of times I don't use a border at all, but choose my binding carefully. mostly in a contrasting color/scale. If I do use a border, each one is no more than 3/8's the block size. If more than one, I often make the inner border narrower, and the outer border wider.
I lay it out and play with it. I start with the fibonacci ratio's, but often take a look at the golden ratio to see how that looks. They're similar but not exactly the same.
No rules...just do what looks good to you. Best rule is: Have fun!