That'll probably do it. I think of jumping as how high a players head gets when he jumps, so i think its really based on their natural height and maybe, in addition, their natural fitness and strength. If a player is smaller than 6 feet tall he really won't be able to challenge for headers that effectively. Sure these players will get on headers well enough if they're open and have a good heading stat, just don't expect them to help out too much on set pieces or floated crosses.