Seems to me that measuring the top first would be the best place to start figuring out how much backing one would need.
From what I've read, length plus 10 inches and width plus 10 inches if the quilt is going to be sent to a long armer to be quilted
Example:
Top is 80 x 90
Backing should be 90 x 100
How one achieves that 90 x 100 can be any number of ways.