Nostalrius Begins - Quality wow vanilla realm (1.12)

It's not at a 1:1 ratio, I see.

Base spell damage + (gear spell damage * spell coefficient)

Long story short, you divide the cast time (before talent / haste effects) of your spell by 3,5 to get the spell coefficient. Which means a Fireball gets 100% of your +Damage, while a Shadowbolt only about 86%.

Lots of other rules, exceptions and shit, but if you don't know anything about spell damage coefficients, this should be a lot clearer now