It also isn't constant between all people. There are some people who can still see every ab muscle at 15% body fat. there are others who can't. You can guess and possibly come pretty close, but that should probably be left to those who have actually taken many body fat percentages of many different people.
More important than anything is the fact that it largely does not matter. It would be utterly stupid for a thin guy to start dieting just because his body fat came back with a high reading even though he doesn't look it. Underwater weighing may be one of the most accurate methods, however.