It's 1 level, so whatever level you are, if you're in range for it to work, you jump to the next level. But it doesn't matter anyway since leveling in the 50s/60s doesn't really take hardly anything anyway, and leveling for the sake of leveling tends to make the game harder for you than it needs to be.

As a free player, leveling is more likely to screw you than help you.

ah ic, i get it now.
thanks for the reply Otsuro

Important distinction is it will set u at 0% exp next level. So if you use it at level 58 with 99% exp. You will be level 59 with 0% xp. All that xp goes down the drain.
Therefore, to maximize the experience gained, use it at level 59 with as little xp gained as possible.