The Mastery level experience is not very user friendly/easy to find. The only place I've seen it is when on the main menu hovering over the Mastery XP circle/orb next to the play button.

I think it would convenient ease of life if there was a Current Level Progress Bar when you click on Mastery and it shows you what level you are working on that way you have an easy sight representation of where you are at within that level.