QUOTE(hitokiri84 @ Jul 10 2012, 23:49)

It's even easier to just list as [next level * 20]
I wouldn't call that easier, it's basically the exact same difficulty as current level * 20 + 20. It might be easier for those that have issues with basic arithmetic, i.e. addition.
QUOTE(varst @ Jul 10 2012, 23:51)

Now I guess for every forge level it gives rises the upgrade cap by 1 for those with 50 levels. But how about the ones with 20 levels cap? (So what's the new cap at 50 forge level?)
Everything is linear. Every forge level increases the cap of destruction/slaughter by 1, so at max FL, those will cap at 100 upgrades. Same for the 20 ones. The cap is increased by FL/50 * 20. Note that this rounds down.
QUOTE(varst @ Jul 10 2012, 23:51)

It also depends on the number of upgrades you've done on the equip, so it's difficult to draw the conclusion.
That's not entirely true. The number of upgrades doesn't directly affect the EXP gained, but rather the quality of the materials required for the upgrade is what determines it. It just so happens that the more you upgrade, the higher the quality of the required materials becomes.