There are a few simple rules (which apply to level 20+ equips):
1. The max stat value is the level of the equip divided by 2. For example:
a lvl 46 hat can have max stat 23, lvl78 pants can have max stat 39.
--note: this does not apply to boss weapons. (e.g. I have a lvl20 boss axe with DEX+15)
2. Different kind of equips can also have different max number of stats, e.g. some can be max double statted, triple statted, quadruple (4) statted or quintuple (5) statted.
max double statted: boots and hats
max triple statted: weapons, shields, shirts and pants.
max quadruple statted: boss weapons
max quintuple statted: blue equips
Okay, that's all for now. If someone can do a little research on the set equips, we can add that here. I know the shirt adds to hp, the pants to sp, and the boots to crit rate, but how much for each level I'm not sure.