I'm sure there will be someone who chimes in with a formula you can use to get a close estimate. That being said, you might want to be careful using Gel-Spun for backing. There have been cases where it was wound onto the spool under pressure (i.e. fighting a large fish) and when it "relaxed" it expanded enough to warp the spool. I'd stick with standard backing, especially since the likelihood of you ever losing a fish becasue you run out of backing is fairly remote; at least in my experience.

Jim Smith