No rules
What I usually do is place border fabrics and quilt top on the floor and position them to audition the amount of border I want showing. I always position showing a full corner and as much as the top and one side as possible to get the full effect. Then I'll keep moving the border fabric to different widths until I get what I want.
I do the same thing to audition different fabric. Sometimes auditioning different fabric and different border widths can take me a full day and I get lots of exercise bending and re-positioning. I need a design wall!!