It's impossible for a person to fully realize their true genetic potential. The calculator is based on elite level bodybuilders. However I think that their estimated FFMI was probably calculated based on their competition weight. For this calculator to be accurate the person would need a reliable way of calculating bf% and I think they would have to have single digit bf%.

If you find your current physique depressing then keep on trucking because an elite level natural physique should be anything but depressing. I figure my FFMI is somewhere between 23-24. Which seems close to 25 but the difference is about 10lbs of muscle...which is a lot.