The blackbird ridge line is 101"...as posted on Brandon's website....I just made a new hammock with a whoopie sling for the RL.
My hammock is 11 feet long. My whoopie sling is 99" to 109" long....I tried it out at 109" first (83% of hammock length) and it didn't feel comfortable enough....I kept adjusting and eventually had it set at 99" which feels great for a diagonal lay.
I used the camo 1.1 that sclittlefield just sold and made mine a double layer. I am 6'3" and 235 lbs (as reference).
I think there are some general ranges, but no true one size fits all when you are making custom gear.