I could put this in the main guide, but really it's not so much a function of Promyvion as it is a function of level capped areas. This was in the release notes, but I'll repeat it, as the question comes up a lot.
If you're a level that's near the cap (say, 30-32 in Promy, 40-43 in Phomiuna, etc.) you may notice that you're earning *far* more experience than your friends. You're all capped at 40 together, but your xp is rockin' while theirs is kind of lame.
How does the exp reduction work if you're over the cap
The game gives you the greater of two values. Either...
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Formula 1) The EXP you would earn if you were your base level
../ OR /..
Formula 2) 1/2 the EXP you would earn at the level-cap.
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There is one extra catch. If you're earning exp via Formula 2 you are not eligible for EXP Chains. Players who are near the cap -are- and will be able to xp chain to their heart's content.
What? I can't do math. Math is hard. ><
Okay, let me break it down for you.
Let's pretend that you've walked into Promyvion and your actual base level is 33. You wind up capped down to Level 30.
You fight an IT Wanderer on the 4th floor. It's IT++ to a Level 30 -- let's pretend it's real level is 38.
Formula 1)
Lv38 beast vs Lv33 PC - 5 level spread, maybe it clocks out as being IT. You would earn 160xp for the battle.
Formula 2)
Lv38 beast vs Lv30 capped PC. Definitely IT. Clocks out to 200xp. Because you're on a capped formula this is cut in half - You would earn 100xp.
Formula 1 gives you the best result, so that's what you get when you walk in as a Lv33 and are capped down to 30. You may participate in xp chains. Fighting in Promy will be a profitable venture for you.
A friend in your party runs a Level 60 WHM and has come along to do the Promy run with ya. He's in the party for the same wanderer battle.
Formula 1)
Lv38 beast vs Lv60 PC - the monster would rate as Too Weak. YOu would earn no exp from it at all.
Formula 2)
Lv38 beast vs Lv30 capped PC. Definitely IT. Clocks out to 200xp. Because you're on a capped formula this is cut in half - You would earn 100xp.
Formula 2 gives you the best results, so your friend who's on his 60WHM capped to 30 will earn 100xp for the fight. He won't be able to participate in xp chains. Fighting in Promy won't be as profitable, but at least the xp he's earning is being applied to his main class.
I hope that helps. ^^