Leveling a second class optimally?

[Niwashi]

When does the 50% increase to exp take affect? It takes affect as long as you have 1 class at least 1 lv higher than the class you are leveling? Right?

Yep. Well, with the caveat that it appllies only to DoW/DoM classes. So long as you're on a DoW or DoM class that's at least 1 level below your highest DoW or DoM class, you get the 50% bonus.

Note: this is different than what the in-game help says about it. That says it's proportional to the number of levels below you are. Actual testing seems to show that it's always a flat 50%. (It looks like they changed their mind during development and one plan got implemented but a different plan got documented in help. I'm not certain which of the two is WAI and which is the bug.)