Dungeons at a higher level award roughly 1/3-1/4th of a level, if you queue for them outside of the roulette. i.e running Sirensong Sea (at level) would award me 1/3rd-1/4th of my EXP bar. I believe that OP wishes for, outside of the daily bonus, dungeons to always award that 1/3rd-1/4th EXP regardless of if you get Sastasha or Sirensong Sea, in a duty roulette.

This way, if you wish to run the roulette ad nauseam, the first bonus is applied (which I think is a bit under half your EXP bar) and subsequent runs of the leveling roulette will always award that 1/3rd-1/4th EXP.

Unless I'm mistaken?