Originally Posted by Elenmir

I looked at my old data again, adjusted some of it (have to be careful with the base values for spells due to the level-based scaling), and collected some more data to get better bounds and verify some corner cases. As it turns out, the formula from wowwiki seems to be accurate for nearly all spells, with a few notable exceptions.

For the spells I've tested, the exceptions to the 'next rank + 5' rule that I've found are:

Greater heal rank 4, 69 (expected 65).
Greater heal rank 5, 70 (expected 68; this is the AQ20 rank).
Heal rank 4, 45 (there's no next rank of this)
Flash heal rank 7, 67 (expected 66)
Renew rank 9, 67 (expected 65; next rank is the AQ20 rank)