here are two great Threads on the subject of Leveling Up with a Hireling and how Experience is affected.
First Perk: you need X amount of Experience for the First Perk. after choosing, you go back to level 1. Second Perk: you need 2X amount of Experience for the Second Perk. then back to level 1. Third Perk: you need 3X amount of Experience for the Third Perk. then back to level 1.
Fourth Perk: you need 5X amount of Experience for the Fourth Perk. then back to level 1. Fifth Perk: you need 7X amount of Experience for the Fifth Perk. then back to level 1. Sixth Perk: you need 10X amount of Experience for the Sixth Perk. then back to level 1. Last Ascension: you need 14X amount of Experience to get back to level 99 for the last time.
when I was Ascending my Wizard and chose my Third Perk, and saw how much Experience I needed for the Fourth Perk, I felt a serious chill up and down my spine and a shock to my nervous system.
here is a post I made while Ascending when I met a Mythic Monster and got lots of Experience.