The operative line for a client casting the spell would be:
Code:
damage += damage * spells[buffs[i].spellid].base[0] / 100;
The value in base[0] appears to be the percentage amount added to the damage, so a value of 50 in base[0] should add 50% to the damage of the spell. As an example for a 50 damage spell it would be 50 + (50 * 50 / 100) = 75 total.
If you're getting other results and this is the only effect being considered then you'll probably have to trace through and see where else the damage value is modified.