Fret 24. It's your last fret, so as long as fret 23 plays OK, then there's nothing to really worry about. Those last upper frets aren't affected by any truss rod adjustments, so just try to get them all as level as you can. if frets 21, 22 and 23 are level, then the blade should slide across to 24 without hitting it. It doesn't matter if the last fret is slightly lower than the others, you just don't want it higher.

If I'm levelling the whole fretboard, I tend to try and put a very slight downward slope (say from 20th to the 24th in your case) on those last few frets so that there's no risk of any bent strings on those upper frets choking out. I find this quite useful on smaller radius boards, though it's less important as the fretboard radius increases and the board is flatter.