Bottom line is to measure your suspension travel. On a stock bumpstop you have to figure that the bumpstop will compress all the way on a big hit on a rut or dip. You want the bump stop to bottom all the way before your shocks bottom. Shocks bottoming is more important than tires rubbing the fenders. If you get too much rubbing you can trim a little more, if the shocks bottom you get an uncomfortable thud and it will eventually damage the shocks or break the mounts.
One way to check everything for sure is to remove the coils and set the front end down on the shocks and bunpstops to see where you are, then adjust accordingly.
Ditto on the Prothane coil inserts, by far the best bump stops for the front of an XJ. If you get them, keep in mind that they will compress quite a bit, mine compress 3+ inches, but it provides a very nice progressive bump stop with no harshness. You buy them long and trim to fit, and they just sit inside the coil. Only modification you need to run them is to replace the stock bump stop with a flat plate for the insert to hit against, you bolt the plate to the tower like the stock bump stop.