Remember when you set up your shocks that you also will surely need bumpstops in the proper place and a possibility of limit straps. This can change the length of shock you need greatly. After you locate your bumpstops to the proper place to prevent the spring from inverting and causing damage, you will probably find that you dont need as much shock as you thought. I would think that 13" of travel would probably be enough on a tuned suspension, with a max of 15-16". Anything over that and you are probably either overextending/inverting your springs, or if you have a set of POS buggy/revolver shackles that you are getting more than usable droop.