Instead of taking 30 levels, it should calculate how much xp is required for that level from 0, in the case of 30 levels it is 825 xp. When you do a level 30 enchant it will only deduct 825 xp now.
What this effectively mean is if you use a bunch of emeralds and get to level 40 (1895xp), you can enchant 2x times at level 30, instead of enchanting once and going to level 10(170xp, a loss of 900 xp for no reason).
Instead of taking 30 levels, it should calculate how much xp is required for that level from 0, in the case of 30 levels it is 825 xp. When you do a level 30 enchant it will only deduct 825 xp now.
What this effectively mean is if you use a bunch of emeralds and get to level 40 (1895xp), you can enchant 2x times at level 30, instead of enchanting once and going to level 10(170xp, a loss of 900 xp for no reason).