It's a flat 50% as long as your current level is less than your highest level. There's no variation in percentage.
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To be honest, I haven't paid enough attention as to whether it's a flat 50% or not when leveling. I do believe it says in the in game help that it's based on the difference in levels. Either way. It's awesome.
EDIT: I do actually recall seeing my bonus diminish as I closed the gap in levels on 2 of my classes. But I think it remained a flat 50% once I got a class to 50.
It has a fairly generous cap too. I swear I got like 15 levels before I managed to deplete it after taking a long break from the game.
Of course, it's also great not leveling anything for a while and logging out in a town or somesuch each time you log for the night. I have yet to see the end of my additional +50% for rest so I get 100% on all kills.
In some of the beta phases, it was based on the level gap. But either in the final beta phase or on release (can't remember which), it was changed to 50% flat. I'd imagine because SE realized the other way would encourage people to do silly things like level to 50 without unlocking jobs or cross-class skills. But the help text wasn't updated to reflect the change. I keep forgetting to check whether it's been updated since.
Tip: You can easily level your first class to 50 by doing nothing but the main storyline, class quests, Hunting Log and any FATEs you come across. (In the late 40s, you may have to add a few leves and extra FATEs, the game slows to a crawl around 45-50). You can safely leave all side quests untouched to come back to when you level your next class. This will make leveling your second and third class a bit easier.
Tip 2: If you know you will level all classes, don't be afraid to greed roll every pink and green (and eventually blue) gear you come across. You will use it later, and you are not taking away anything from people who need that gear right now, because Need rolls have priority over Greed rolls.
This game is honestly too massive for multiple characters. You probably should stick with one.