OK I was going to pot this in the elemental staves thread, but it got locked.
Stoneskin absorbs a certain amount of damage from physical and magical attacks.
The amount of damage absorbed depends on a forumla
The Formula on wiki
So the equation is based on MND and Enhancing skill ONLY and the amount absorbed has a hard cap of 350HP, which means there is a point where adding more will not make it stronger.
There are 2 items that can raise this hard cap, but we ignore them for now.
From the equation we can calculate the total MND needed to cap Stoneskin for each job with native enhancing skill
Job Amount of MND to reach the Cap
RDM 95
SCH 95
WHM 104
PLD 110
BLM 114
Now for RDM, SCH and WHM reaching their caps are pretty easy, depending on race and subjob the amount of MND needed from gear is likely to be 25-40.
PLD can reach 110 MND, but this thread isn't really to do with PLD so I will skip this.
BLM has a harder time of it, they need a lot of MND to reach the cap and suffer from low base MND so they are likely to need 50-70 MND. This is likely need a lot (if not all) of your gear slots to acheive.
The other thing we can deduce from the equation is:
3 Enhancing skill = 1 MND when trying to reach the 350HP cap.
So something like Warlock's Tights are effectively 8MND (15/3 = 5)
So if you are job that has acces to AF or relic that improves Enhancing skill, it maybe prudent to use these items if their skill is more than 3 times that of the equivalant MND peice.
Once you reach the cap of 350 adding more MND or skill is pointless, maybe a few extra mind incase you get hit by Shock (or other MND down effects).
So you could declare you stoneskin set complete once you reach a level of MND/skill that you are happy will cap you in nearly all situtations.
But there is more you can do. Stoneskin has a long cast time (10 sec) and a long recast (30 sec), so one thing you could do is use some fastcast and haste items to get the cast/recast timers down.
Obviously if you start using slots for fastcast or haste you will lose some MND, so you need to check your calculations each time you remove a MND item.
Another thing you need to consider is, you cast stoneskin becasue you are getting hit (or think you are going to soon), so you need to make sure the spell isn't interupted. Stacking 'Spell interuption rate down' isn't overly effective (from my experience) I find equiping 'Physical damage taken -x%' is the best way to ensure you cast without being intereupted. So I cast in a Terra's Staff, not because it makes stoneskin stronger, but because it keeps me alive long enough to finish cast the spell.
With all that said here is my stoneskin set, it is by no means perfect. It needs a loq earring (in the empty ear slot) and if I can get a bit more MND a few more slots I could use my Dusk feet +1.
Currently I can cast in 6 seconds (40% off the 10 second casting time, 20% from traits, 20% from gear)
Currently I can recast in 23 second (23% off the 30 second recast time, 10% from traits, 13% from gear) (19 seconds if have haste up)
Hopefully that hasn't patronized too many people and maybe has give some an insight into stoneskin