If you do what #2 said, then subtract 5/16" from the measurement. The point is to get to the type of measurement that the first guy told you: tip of ball to bottom of cup ==> effective length... for SOME manufacturers.
You gotta look at the manufacturers specs to see how they spec them. Forget a distributor's data: go to the manufacturers data sheets. In the case of Comp, you have to look at their comprehensive catalog. For the 7821's they list the length as 'overall length".
