Requirements are highly individual and sort of subjective. If I ate 40 grams of protein a day for some period of time--eventually, I'd probably be in protein balance, likely after losing some amount of lean mass. Take 50 people just like me at that point, do a nitrogen balance study--you might find our protein requirement to be 40 grams a day. Another group of teaser clones eats 120 grams of protein a day for a similar period, and their protein requirement by nitrogen balance, after lean mass increases and plateaus, is 120 grams of protein a day. The whole exercise is a little meaningless, until you develop some sort of opinion about what level my lean mass should be at in the first place.