http://wiki.ffxiclopedia.org/Stoneskin
In a nutshell, +3 enhancing = +1 MND as far as improving Stoneskin goes. 3 enhancing/1 MND mitigates 1 damage at first (i.e. with low numbers), later 2 damage, and still later 3 damage with high numbers. And the max mitigation is 350.
Using Stoneskin while equipping a Stone Gorget will add 30 mitigation to whatever would otherwise have been calculated (this boosts past that max, by the way, so with Stone Gorget the max is then 380).
EDIT: Assuming no +enhancing skill gear (and many of these jobs *do* have some):
RDM/Whatever (B+): 256 enhancing => 95 MND required to cap
WHM/Whatever (C+): 230 enhancing => 104 MND required to cap
PLD/WHM or PLD/RDM (D): 210 enhancing => 110 MND required to cap
BLM/WHM or BLM/RDM (E): 200 enhancing => 114 MND required to cap
Other/RDM (B+ at 37): 109 enhancing => 144 MND required to cap
Other/WHM (C+ at 37): 105 enhancing => 145 MND required to cap
RDM and WHM shouldn't have much trouble capping Stoneskin with gear they haul around anyway. BLM can pull it off if they bring a whole bunch of MND gear for the purpose (they can certainly get it high with a few easy macro swaps). PLDs may or may not want to bother hauling around MND gear for when subbing WHM or RDM (seeing as they take relatively low damage anyway, being PLDs).
Other jobs subbing WHM or RDM should probably just not expect to cap Stoneskin, but they can haul around as much MND as they like to improve it as best they can.
