That would be very hard to do. Most people don't keep records of other people at different body fat percentages. That is why general statements are made. At 10%, MOST people should be able to see their ab muscles.
Above is a picture of Bob Sapp:
In that pic he is probably around 10-11% body fat. That is just an educated guess based on taking a good number of percentages over a few years as a PT. Chances are, he isn't below 10% and he may even be closer to 12%.
At around 8%, you are fairly lean but no striations are usually visible until you get closer to 5-6% which is what most bodybuilders compete at, especially on the NPC level. That level usually isn't easy for most to maintain and still make much physical progress.
On the other end, 15% is usually about the point at which all of your abs have taken a hike and you are looking pretty filled out. 18-20% would cause most smaller people to pass for "chubby" and more muscular people to get described as "stocky".
Everyone carries weight differently, however, as I had one frat brother who taped out at over 16% body fat, but every ab was visible and there is no way anyone would have guessed him at more than 10%. That is why the number means so little. It doesn't matter if you tape out at 10% body fat...if you have no muscle and it just doesn't look good. The number means shit for most people. They are better off going by the mirror.
