Of course that's true but yeah. Or calculating as if you have level 20 items dropped from a certain floor and for the power 103, hp 103 or MP 103, it turns into 19 or 20 points or however many.
For base DMG of level 20 items, you either look up in forums, find a level 20 weapon yourself or calculate using the level formula to find the exact base DMG of an item at a lower level if you're good at math .
Then there's clues in the arena rules. Mythstones are nerfed by 70% in arena which is an easy calculation. Everything else you can find by checking level 20 character or by calculating. Stat caps are reduced by 60% for everything which is another clue.
Damage reduced by 95% is a big one however. Normal level 20 character in campaign or dungeon crawler mode will have more DMG than an arena character . For DMG, you just reduce it by 95% once after calculating DPS formula (that's my easy way to see how it works).