i usualy think potency is directly related to w/e stat and acc (also called duration) is related to skill... but that would be too simple.
i hear brds complain about having +obscene ammount of chr but it doesnt improve acc, and that would make sense is +chr = +potency, and the acc only comes w/ skill. Also would explain why elegy is so strong (because most brds max chr in hopes of acc).
for rdms, i think its the same also, skill for acc (resists/duration) and stat for potency. one example of stat effecting potency is when compared to hojo: ni. With low mnd, hojo will overwrite slow.. but with stacked +mnd, slow will overwrite (and is more potent) than hojo. i have one of grendels video's that actually shows how +mnd makes slow actually slow the mob more than without +mnd (rather dramatically). Seems that balance everyone needs to find is, do you want an enfeeble that sticks but is weak, or do you want an enfeeble thats hard to stick but very strong.
for whms, mnd has a huge effect on cure5 and stoneskin. the other cures all cap, and no ammount of +mnd will make them any more impressive, but with cure5 +mnd makes a huge difference. Max cure5 ive done is near 1000 (without divine seal). i think you can break 1000 on lightsday w/ some effort but i personally havent tried. without +mnd or potency gear, cure5 is around 600 i think.. forgot since its been a while.
mnd's effect on stoneskin is seen here..
Code:
F1: ([Enhancing Magic Skill] / 3) + MND when X < 100
F2: {([Enhancing Magic Skill] / 3) + MND} * 2 - 60 when X = 100 - 150
F3: {([Enhancing Magic Skill] / 3) + MND} * 3 - 190 when X > 150
"X" is calculated by
X = ([Enhancing Magic Skill] / 3) + MND + 20
which means for whm and rdm, each point of mnd increases potency of stoneskin by 3 points.
As far as nuking, i would think the same applies (although it still seems too simple, and im sure its not this easy). +int = +magic attack bonus = potency, while +skill translates to resist rates.
Ive heard a lot though, that BLMs get less resist rates when etuded (which is only +int, so in theory only +potency) so im sure all these formula's have more variables than we know about. maybe something to do w/ skill effecting +stat caps.. but i havent ever seen anything concrete about that, with real numbers.