I'm not saying 50% is the right number, but I can imagine that a 100% requirement would produce very noisy results, while 50% might achieve a much smoother grap

I'm not saying 50% is the right number, but I can imagine that a 100% requirement would produce very noisy results, while 50% might achieve a much smoother graph that produces a (hopefully) better view into the future. It might be reasonable to look at where a 95-100% result appears relative to the 50% result for past evaluations, and then project out from the projected 50% results in the future by the same amount -- but again, it might be the case that looking for 100% results relative to 50% leads to a very noisy result that can't be generalized into the future well.