Ok imagine you have two 20-sided dice:

Dice 1 is your craft type.
1-4 = fail
5-10 = +0 low
6-15 = +0 high
16-19 = +2
20 = pc
*note: it's pretty much 50-50 for what the actual type is (like rate or cd)

Dice 2 is the slots:
1-19= +0 slots
20 = +1 slot (except for 2 slotted crafting items, they cannot increase)


Sounds about right from my 3.5 years of crafting experience. Although, i would say that as the grades go up, it's larger dice than 20, so the chances of pc lower, and failing is increased.