Is the "Armoury" bonus XP for other classes level-dependent or a fixed 50%?

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.

However, many places (including another answer on Arqade) claim that's incorrect and the bonus is always a fixed 50%.

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.

And here:

The Armory XP bonus caps at 50%. It's possible to hit this cap with a relatively small difference in levels.

And here:

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?

As of patch 3.0, the experience bonus is a flat +100% when on a job below level 50, and +50% between levels 50 and 60.

From the 3.0 patch notes:

As always, this bonus is only active if you're playing a class/job that is lower level than your highest-level class/job.

It is a flat 50%. I believe that the older comments that allude to it scaling up based on level are either based on 1.0 information or possible design that has since changed.

(Source: Personal experience in 2.1 and onward, both at sub-50 [Arcanist 30, leveling Conjurer and Thaumaturge] and at-50 [Arcanist 50, leveling Lancer and Archer].)