To find out what height you want to place the bumpstop can, you'll need to compress the suspension until only 3/4 inch of the shock's shaft remains. This will be the point of maximum compression for the hydraulic bumpstop, and you'll want to set and weld the bumpstop can to hold the bumpstop accordingly. Why leave 3/4 inch of travel in the shock? The shock is in a different location in the arc of the suspension travel and that extra little bit will be utilized during axle articulation. Once you find the desired position of your bumpstop can, cut off the unnecessary upper portion (which you will have unless you have 12 inches of lift on your XJ).