How should I scale magic damage in my RPG for balance and wow factor?

Should it scale linearly with spell level, so "Fire 2" should be exactly twice as powerful as "Fire 1", or what? How would you do it?

Example:

Fire 1 might do 25 dmg plus your Intelligence Attribute plus 2d6.

Fire 2 might do 50 dmg plus twice your intelligence plus 4d6.

In this case, the projected Fire 2 damage is probably more than enough to kill most enemies in one hit. I'll need to test it, I guess.

How should I scale magic damage in my RPG for balance and wow factor?
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