A while ago, lostmyth2 put out info on "everything he knew" abou the time he left the game. In that, he thought one full letter grade amounted to about a 1.7 difference, with the effects being cumulative. For example, an A would have a 1.7*1.7= about 2.9 advantage over a B, a 1/3 letter grade advantage would give about a 1.2 advantage, and a 2/3 advantage would give about a 1.4 advantage. And so on. He also thought there could be a difference in prestige of A+ programs, depending on the relative results to achieve it, along with baseline. Hope this helps.