I would say that the only way of being 100% American, wouldn't be to trace back 2K year, but to actually trace back to the original Diverging genome, based on location, and to then go forward until today, with the same essential bloodline with that genome change. (By the way...don't hold your breath)
Humanity actually functions better by NOT being 100% anything. It is better to incorporate many of the adaptations made by man into a single conglomerate, for the betterment of the species. Actually, scientist should bloody know that by now, so taking ones own genes to produce another human would be asinine even to them.