The following calculations apply to both Stamina and Magicka DPS.
Take note that the ingame tooltip for the Shadow Mundus is wrong as of v6.2.5. It is NOT 17% with all seven of your armor pieces being Divines but rather 18%.
Crit damage assumed: 0.5 (base) + 0.1 (Minor Force) + 0.2 (Major Force) + 0.1 (Minor Brittle) + 0.19 (Elfborn/Precise Strikes) + 0.09 (Catalyst) = 1.18
x/1.18 = 0.556
x = 0.556 * 1.18
x = 0.65608
x/1.28 = 0.556
x = 0.556 * 1.28
x = 0.71168
x/1.38 = 0.556
x = 0.556 * 1.38
x = 0.76728
x/1.05 = 0.556
x = 0.556* 1.05
x/1.15 = 0.556
x = 0.556* 1.15
x = 0.6394
x/1.25 = 0.556
x = 0.556* 1.25
x = 0.695
Basically every class except Dragonknight DPS will most likely be using the Shadow Mundus Stone in raids.
Assuming your Spell or Weapon Critical rating can reach ~68% or above you should be running the Shadow.
Assuming your Spell or Weapon Critical rating can reach ~58% and below, you should be running the Thief Mundus.
If your Weapon or Spell Critical rating is somewhere between ~59% to ~68%, we would still recommend going with the Shadow Mundus.
Of course your Critical chance will differ depending on sets, so if you're not using Mother's Sorrow or Medusa for example on a Magicka DPS character you will most likely go with Thief rather than Shadow.
Moreover, while it's arguable that you would go Thief on Necromancers including Elemental Catalyst Magicka Necromancer, in optimized raid settings you will most likely still go Shadow due to the increase in your Critical chance once the boss hits execute range but this can of course depend on the percentage of your fight time spent in execute/before execute (i.e your group/fight).