I was off by 4 inches short the first try, went 2 inches too short the second try. The third try it was too long by about an inch and a half. I figure can use the second and third together. Each side is about 20 feet, so if the sprit is off to one side by 1 inch in 40 feet it's close enough...

The problem is when I do the back-stays or other rigging it will be a different diameter rope. Each splice is taking about 20 minutes at this point (the first took more than an hour but I've done 6 now so I'm improving ).

Without a formula -- the next time I do this I'll mark the line about 6 feet away from the first splice, then measure before and after the tuck so I know how much shrinkage to expect with that particular rope.

PS -- I used a spreader bar clamp to tension the rope to try to pull out some loss, and it didn't seem to recover any measurable amount. I cut the taper by cutting off one strand every inch for 12 inches.