Is there a mathematical forumla or anything?

"o calculate the experience required beyond Level 10, multiply the experience you needed to level up the last time around by 1.5 and round down. The resulting number is the experience you need to get to the next level. For example, as a Level 10 Thief, you needed 122,171 XP to get to that level. Take 122,171 and multiply it by 1.5, giving you 183,256.5. Rounding that down, you need 183,256 XP to reach Level 11. If you multiply that latter number by 1.5 and round down, you will then determine the experience you need to reach Level 12 (274,884)."

http://www.uesp.net/wiki/Arena:Classes

Many thanks, my friend!

You are welcome!