Magic in Flight is complicated under the hood but simple for the player to understand. Access to spells is gated by level, spells fall into various types, etc. You have basic spell power, spell critical chance, and spell critical multiplier. ALL spells benefit from these; for basic damage-dealing spells this simple increases the amount of damage done; for more complex spells this actually opens up additional effects.
The core spellcasting system is tied to the Mage class, which spells you have access to is governed by the number of mage levels you have. The idea is to make "splashing" a couple of levels of mage to provide some spellcasing ability possible, while still making core mages very viable.
Let's start with the most basic of all magic spells: fireball. Every self-respecting game with magic includes this. So you have Fireball, and for the sake of simplicity, let's say Fireball does 4d6 damage(24-36). Since this is "magical fire" it deal damage directly to hit points.
For the sake of simplicity, let's give it the type "elemental" rather than making "fire" a specific subtype. We will start with a base of 100 spellpower, which means the spell does "100% damage" EG a minimum of 24 and maximum 36. Now let's say the player manages to get his elemental spellpower up to 110. The spell now deals "110% damage" meaning scaled by 10%, or 26.4 to 39.6. Obviously there will be some rounding, but that's the basic gist of spellpower.
A spell is an attack like any other, and as such can hit critically. The key difference is, unlike combat, each spell-type has an independantly-tracked critical chance. Your basic spell crit-chance is 5%. So if the spell successfully hits, the engine will check and see if it was a crit, and apply accordingly.
The critical multiplier determines how much extra damage is dealt. Basic multiplier for all spells is 2, so on a crit the spell does double damage. Using our example above, "Fireball" would deal 48-72 on a critical hit, and 52.8-79.2 with 110% spell power. 3x multiplier triples it, etc.
Sources of improving this
Ideally, the basic mage class gives you access to spells, but not a lot else. Spellpower, spell criticals, and critical multipliers come mainly from perks, equipment, and other sources. This would allow say a Warrior to take 1 level of Mage to get access to Fireball; then be able to hurl an extremely powerful fireball with the right equipment. His trade-off for getting that fireball is all the things he doesn't have room for because he wanted to hurl fireballs.
Ultimately, a pure mage will have access to a lot more spells, but the individual spells should not be a lot better than someone only splashing a few levels of Mage.
High spell power and high criticals give spells additional effects. In keeping with the Fireball example, a fireball with a power of 300+ could knock down targets on a critical hit.
Each spell falls into a specific type. These are also magic damage subtypes.