The way the mod currently works is that, when pressing "add level", it removes 5 levels from your XP bar. This would be fine, if levels were not exponentially more expensive. Level 5, costs 55 xp points. Level 40, costs 2920. That means for 2920 XP points, assuming you spent it at level 5 instead of level 40, you would get 53 levels, instead of the 8 you get if you start converting at level 40. This means that the more XP you have, the less efficient converting becomes. I believe this is not intended behavior, and instead should remove a specific amount of xp points, not xp levels.