From what I see, at 22nd level, it's the level 18 XP value doubled, then 150 xp added. At 23, the additional xp is 300, 450 at 24, 600 at 25, 700 at 26, 800 at 27, 900 at 28, and 1000 at 29 and higher levels. I have no idea where that progression comes from, but it seems there is some thought...