How do I determine which bag awning size I need if I already have an awning rail/track on my camper?

Here is an example:

If I have a 13'6" awning rail/track on my camper.

The fabric width on a 13' awning is 12' 8" (fabric width is 4" shorter than the size). The next larger size is 12" greater and that would leave 2" of awning fabric unsupported by the awning rail. This is not acceptable since the tension in the awning rafters would eventually shear the fabric at the rail/track ends. So the 13' awning is the largest acceptable size for that rail/track length.