Spells are absolute precision. They will create the identical effect every time they are used. They can not be modified even in the slightest - a spell designed to harm will always harm, it can not be used for something else.
Their advantage is that this precise effect and the ritual to get it done can be studied, over and over and over, until the spellcaster has become so fast that it takes seconds instead of hours to complete it.

The Power Level of a spell defines its magnitude and difficulty. It depends on three factors:

Effect

1. cosmetic effect with no real consequences for the target
2. a small effect, like a minor wound or a nagging voice in your head
3. a large effect, e.g. a serious wound or a strong desire to do something
4. a severe effect, instant death or irresistable control
5. a massive effect, like being instantly turned to ash or total control and memory wipe

Target

1. a small item or a part of a human-sized being
2. one human-sized being or a large object
3. a small group (5 or less) or a tree, small hut, large waggon, etc.
4. a large group (up to about 20) or a house, a boat, etc.
5. everyone around (~100 people or so), a whole field, ocean-going ship, an entire village, etc.

Duration

1. an instant
2. a short while, about a minute
3. about an hour
4. a whole day
5. several weeks, maybe a month

Multiply all of these together to get the power level of a spell. Note again that casting spells is an art of mind and intent, not mechanics. Magically slicing open a throat is a level 4 (death) effect, not a level 1 (minor wound) effect, even though the wound itself is small. It is the effect that counts, not the means. Likewise, it's a target 2 (one human) effect, not a target 1 (part of human) effect, because it is the human that dies, not his throat.
This is in part a rule to limit "gaming the system" by coming up with "creative" spells.

Examples:

• The typical Fireball may be capable of killing (effect 4) a couple of people (target 3), but it lasts only a moment (duration 1) for a total power level of 12.
• Making the king (target 2) your puppet (effect 3 - we don't want 4 because at that level it would be too obvious) for the month (duration 5) comes in at a heavy 30. If you only need the control for a short while, say so he passes the judgement you want on a case before him duration would drop the 2, making the whole thing a lot more affordable at power level 12.
• creating an illusionary (effect 1) village (target 5) for and hour or so (duration 3) until your hunters think they've taken the wrong turn and move elsewhere comes in at 15. Note that this is a cosmetic effect only, they could, for example, walk right through the walls. If you want the village to be solid, you'd have to go for at least effect 2, doubling the cost to 30.
• Tearing down (effect 4) the city gate or a tower (target 4) might only take an instant (duration 1), but it still has a power level of 16.
• But if you're hungry after all that casting, incapacitating (effect 3) a rabbit (target 1) for a minute (duration 2) until you can catch it comes in at only 6. The alternative, killing it outright (effect 4, but only duration 1) is even cheaper at 4.