I perfomed a quick calculation on pull distances and it agrees with you - 3" of pull draws the sail down .125"...that's 8:1. I've tried every conceivable way to politely disprove what you've stated but keep coming up with your answer. Where I keep getting confused is that if you consider one of the blocks that terminates the blue line is free and not stationary - you might think that it adds to the tension on the blue line. I was considering that this effect was putting tension on the blue line equal to 4x (2x + 2x = 4x) because it was being pulled on either side by 2x but, as you stated, that isn't possible - if you split the system in the middle, the sum of the forces doesn't equal zero if you look at it that way. I'll drop a line to Murrays to let them know that they should probably re-examine that 16:1 description (althought they leave it a bit cloudy with the word "theoretical" thrown in there)

If you break the following diagram anywhere, your forces are not 20lbs in the middle but always 10lbs. i.e. the sumation of forces in a stationary system must equal zero no matter how you break it down.

10lbs <--------------->10lbs
10lbs <-----/10lbs (not 20 here).