Click the button to get more EXP. Click the "see inside" button to see the exp. The max level should be the highest level you can obtain in your game. The factor is how fast you should be able to level up, with a lower number if you should be able to level up faster, and a higher number if you should be able to level up higher. I do not recommend using a factor over 5.
Someone might have used something like this before me, but I'd like to think that this is a somewhat original idea. Basically with this, the player's exp requirement should increase rapidly when they are initially getting exp, but the requirement will increase slower and slower as the player's level increases, to prevent insurmountable exp requirements.