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If you're getting the JRI 13" B's the shock stroke is 92mm which translates into roughly 92/25.4=3.622 inches. The shock sag should be set at 1/3 of 92mm which works out to 30.66/25.4=1.207 inches.
It makes setting up the shock sag a whole lot easier if you go to your local Harbor Freight and buy one of these or order one of these for $10 bucks.
One you start adjusting your preload you will need to measure your springs to make sure each shock's preload is the same. It also makes it alot easier when you keep going back and measuring the length of your shocks (per Howards suggestion of measuring bolt flat to bolt flat).
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