Editing Scholomance (section)

=Spells=
Now, how to cast a spell. When you're casting a spell, you must determine the magic cost. Some good things to consider for that:

* Range - How far you want to the spell to go.
* Area - How large any area or how many people you want to be victim to the spell.
* Duration - How long you want the spell to last.
* Power - This is the most complex part. How powerful the effects of the spell are.

Each of these is basically pretty arbitrary, just use whatever seems appropriate. Some of these may be 0, but power will probably always be something to consider. Once you and your Head Master have decided on the value of each of these, add all four values together. This is the base magic cost of the spell.

Next stage is Mitigation. You Mitigate or reduce the magic cost of the spell using items and a time span spent on the spell. You name anything you think might be relevant. Blood, bones, trinkets, makeup, symbols, symbolic items, chanting, ingredients, foci, casting time, whatever. The Head Master decides how many points of magic each item mitigates. Your Head Master decides what School the spell fits into and also adds any Mastery points you have in that school on to the Mitigation.
Finally, adding up the total Mitigation, he subtracts the Mitigation from the magic cost. The magic cost cannot be less than 1, even if you mitigate into the negatives. This gives the final magic cost that the caster has to guess. He can guess correctly, he can overpower it (guess high) or he can underpower (guess low) it to variable degrees.

Mastery effects the range in which the spell is correct. If you have 0 Mastery in that spell's school, you have to guess the cost exactly.
If you have 1 Mastery in the spell's school, you have to guess the spell's cost plus or minus 1.
If Mastery 2, the spell's cost plus or minus 2.
You get the picture.

The only way to defend against spells is by using wards which are added to the base cost of spells against you. Wards seem to help only against a single School of Magic. They start with +1 MP and can get up to +5, if a master in a School of Magic is just a single Point behind the perfect guess the spell is now considered underpowered in case of a 5+ ward.