I remember testing it. The global enhancement strength is equal to a number of extra gems put in the spell + number of extra level (above required) of the caster. I haven't identified any random element, but that's a random element - one is not guaranteed to detect it by a serie of experiments
I'm sure that if there's a random element, it is not large, so dispel with 969 extra gems by a decent astral mage dispels pretty much anything worth dispelling