There are some guides online about what is reasonable to expect for mass gain. Probably a link from here if it isn't mentioned explicitly (a while since I read).
While I agree that most people have a genetic muscular potential that will naturally hinder their growth someday if they stay natural, the downside of this information is that, well, it's a dream killer. Bodybuilding and straight training is all about reaching your physical potential and going pass it, which takes years of effort, investment, and time. That weeds out people that want to get bigger because they can say "what's the use, I will NEVER be big like my heroes and my inspiration."
The message it sends, although I am certain it is NOT the aim of this article, is that unless you are a genetic freak you are condemned to be average and should content yourself with peddling your genetic ware. That ectomorphs will remain scrawny (but slightly more muscular) twigs, and so on. Like when you reach your "optimal" potential you will magically stop growing muscle and start training for zero gains.
But how can you know if you don't reach that potential first? That's why I'm against letting this information go around and totem is as being "realistic aims". I'm sure it is, yet sometimes the minuses of overthinking this shit overpasses the pluses, and any information that can kill a would-be lifter's drive to improve him or herself should be given VERY sparingly, until he or she is ready to accept the average limits of natural bodybuilding when he reaches these potentials.
Besides, 190 pounds at 10% may not be massive in a roid gym or a bodybuilding stage, but in a bar, a night club or in the streets compared to the average joe it's friggin' huge. Remember that most people have next to idea to evaluate muscle weight compared to fat weight, and evaluate what 190 pounds at 10% looks like in real life.
The hell with these numbers, I see. Neither me nor anyone will know their potential "limit" until we reach it, then they can start worring. I use these numbers as the expected weight which, at 8-10%, I should on average start having a lot more difficulty gaining muscle mass. But it will never truly stop, only slow down to such a point that you'll have to train months to gain even 1-2 lbs of muscle mass.
Yes, it is true that you will never be above 240 lbs with a six pack naturally unless you are a freak, very tall, or on roids, and that roids are very prevalent not only on stage, sports and wrestling, but among Hollywood actors as well. At lot of the so-called "muscular" actors of Hollywood resort to steroids to gain enough muscle weight in a very short time, then work like crazy to lose fat and look ripped for an audition. One should be reasonable enough to know that these "targets" can take years if not decades of work, and the aid of both legal and illegal supplements.
However, that doesn't mean that we should kill the dreams of other because of mathematical formulas that remain to be scientifically demonstrated with a reasonable sample and rigorous controls.