I always have to do the math - and remember that when quilting , your finished product is going to "shrink" up. My mattress is 16 inches thick, so standard sizes don't always work -
I don't have a footboard, so the end of the bed and sides can be seen. I'm pretty anal, so I would have to have the same border on the end as on the sides. If I needed fudge factor, it would have to be along the top where the pillow shams will sit after the quilt is finished.
I will usually figure/re-figure the borders out when the center of the quilt is completed - double checking the math.