PRE 2.3:

01-60: 4 084 700
60-70: 6 762 300
Total: 10 847 000

Half: 5 423 500

Subtract the 1-60 from our half:
5 423 500 - 4 084 700 = 1 338 800

Subtract 60-61:
1 338 800 - 494 000 = 844 800

Subtract 61-62:
844 800 - 574 700 = 270 100

So, the halfway point pre-2.3 is 270 100 XP into level 62 (43.96%).




POST 2.3:

01-60: 3 378 400
60-70: 6 762 300
Total: 10 140 700

Half: 5 070 350

Subtract the 1-60 from our half:
5 070 350 - 3 378 400 = 1 691 950

Subtract 60-61:
1 691 950 - 494 000 = 1 197 950

Subtract 61-62:
1 197 950 - 574 700 = 623 250

Subtract 62-63:
623 250 - 614 400 = 8850.

So, the actual, corrected halfway point is 8850 XP into 63 (0.0136%). My problem before was that I just used 61-62 and 62-63, forgetting about 60-61. My bad on that one.

That's rather depressing.

Numbers from WoWWiki.

EDIT: Number corrections and formatting.