Neoprene and bungee are both pretty stretchy. It's the stretch that allows it to make a tight enough seal to keep out water. What I would suggest is to throw out the measuring tape and grab a hunk of your bungee cord and take it out to your kayak. attach one end to the outside of the rim with a spring clamp and run the cord around, pulling it tighter until you get a tension you're comfortable with, and then mark where the bungee meets its other end. That's your length. Then add to that 8-10", so that you can tie it to the exact tension and still adjust it for SOF coamings that are smaller or larger than yours. This is one of those situations where fretting over numbers, just because there are numbers to be had, will make things a lot harder in the long run. In the end what matters is a good fit.

m