Sort of yes, I don't know if 1/3- 1/4 of a level is accurate though. Usually your highest available dungeon grants about 1/6 of your XP bar ish iirc.
But reguardless of the amount , you seem to get the idea. Every dungeon run via leveling roulette would provide that same percentage.
I’m currently leveling through my classes in preparation for EW. As of now, every dungeon has been giving me about 1/3-1/4th a bar. I can’t remember if this changes with 60-80 as I’ve been stuck at lower levels ahah.