Spell Damage Increase
Spell Damage Increase is an item property that, surprisingly, increases the damage of any magical attack cast. It is capped at 15% for PvP, but this 15% only applies to the actual spell damage increase property found on some items. For PvE there is no cap on spell damage, so pack it on. There is no formula needed for Spell Damage Increase because it is a true conversion, a 10% increase means your spell damage is increased 10% and so on.
Calculating Spell Damage
A character's skill level of evaluating intelligence is the first thing that comes into play in calculating spell damage. The formula for how much spell damage your getting from your Evaluating Intelligence is this: ((evaluating intelligence * 3) / 100) + 1
The next factor involved in spell damage is intelligence. The formula for this is 1% spell damage increase for every 10 points of intelligence, or (intelligence / 10) and drop the decimals.
The next factor is the inscription Skill bonus. This bonus only takes effect when a character has achieved the level of Grandmaster or higher in Inscription. This is a 10% damage bonus for Magery Spells.