First, you must know was spellcrit actually does.
When a spell crits, it does:
1. Maximum Dmg of your Spell + 1.
Lets say you have Spell Thunderbolt VI, which ranges for 2000-4000 Dmg in your Tooltip.
If this Spell crits, it will always be 4001 Dmg at least.
2. The Dmg dealt is boosted for another 30%.
Honestly I'm not sure on this, but i believe the received boost refers to the dmg the hit would have if it would NOT have critted.
As you may have noticed, crits also vary in in their actual overall dmg. According to the above mentioned 2 basics, the Thunderbolt would always hit for 4001 + 1200,3 (30%) = 5201,3.
It does not, even on training dummy where you can leave aside dmg-modifying variables, like debuffs etc.
I never saw a devs statement on this, though there may be one.
So, only only idea i have is, that those 30% refer to the original dmg.
In summary, a crit calculation would work like this.
1. initial dmg random between 2000-4000
lets say: OriginalDmg = 3000
2. Check if crit y/n
Note on this:
With your gear (and AAs) you accumulate a certain amount of crit, which you can view in your personal stats window (press 'P').
This amount only refers to EVEN CON (white) mobs!
Orange-con raid-monsters for instance are actually superior to you, and therefore you'll never have all your spells critted when you actually have 100% crit. This is why raiders often aim for around ~120 crit.
This applys other way around too, on blue-con mobs you will crit more, but mob will not crit often on you.
Back to example, lets say our spells crits.
BaseDmg = 4001
3. calculating outgoing overall amount
critbonus = originalDmg / 100 * 30 => 3000 / 100 * 30 => 900
outDmg = Basedmg + critbonus => 4001 + 900 => 4901 dmg
4. overall dmg dealt in offset to mobs resistances/defense
after outgoing dmg is calculated, the hit still has to pass mobs' defense which is actually incalculable, bc we don't know mobs resis and mitigation etc.
On summary you can say that 1 crit gains you slightly more than 0,3% more overall dmg.
Slightly more, bc spell always does maximum dmg.
Due to variety of spells/your class and your playstyle, there's no exact valid calculation on this.
But I hope that helps though.
PS: I don't claim that my thoughts and experiences are complete and fully covering all essentials, so corrections may have to be done.