According the in-game docs, the "Armory bonus XP" for lower-level classes depends on the level difference between that class and your main class.
A quick Google search gives a lot of conflicting information. For instance, here:
It is a flat 50% bonus to all but your single highest DoW/DoM class. The pop-up that explains the armoury bonus does say the level difference determines the bonus, it lied.
The Armory XP bonus caps at 50%. It's possible to hit this cap with a relatively small difference in levels.
It's 50% bonus if they're > 2 levels lower than your highest, if they're < 2 levels it's 20%.
None of them provide any sources.
Does anyone have any authoritative source on the matter?
Or better yet, has anyone Used Their Science!!!™ to figure it out?