when to start a new class

[Naryoril]

The official recommendation is to get your main class to fifty first. You also get bonus XP when leveling another class based on how much level difference there is between it and your highest (meaning you get the most XP doing stuff on a second class if your first is already 50). If you're 30+ and not using the associated job with a class, though, many people will fuss at you over it. They're wrong, but it is indeed common. Ultimately, though, the answer is when you feel like it. You can freely switch back and forth. =)

Isn't it a flat 50% xp bonus as long as a different class has a higher level?

[Niwashi]

You also get bonus XP when leveling another class based on how much level difference there is between it and your highest (meaning you get the most XP doing stuff on a second class if your first is already 50).

Isn't it a flat 50% xp bonus as long as a different class has a higher level?

Yes, it's 50% regardless of how much higher your higher class is. The help window describing it says (unless they've fixed it by now - it's been a long time since I looked) that it varies according to how many levels difference there are, but people who have checked at various levels found it's always 50%. It looks like the kind of error that can happen if the devs changed their mind at some point during development - one plan got documented while the other plan got implemented.

So anyway, it really doesn't matter whether you get to 50 before or after bringing up any other classes you want to work on. Wherever you're at, one of your classes will always be the highest and all the others will get the 50% bonus. (All the other combat classes, that is. This doesn't apply to crafting/gathering classes.)