I like to apply loose physics to magic. 

In my current game, I'm using a pattern-based magic system: certain materials under certain conditions will create a specific result. Like in the way that a harmony of heat, fuel, and oxygen can create fire. Certain natural patterns are capable of creating energetic reactions. Certain rudimentary totems (what I call them) are two-dimensional patterns that can be created with simple lines in dirt or paint on a wall. Other totems are three-dimensional shapes, like knots of copper wire in intricate weaves, or a complex spiral of blown glass to a precise volume. Generally, the more complex the pattern and rarer the material, the more powerful the effect, though this is not always true.

In this way, a character must have the right materials and the right items to create the right sort of magic. 

And I like the magic systems that adheres to "...equal and opposite reaction," so your example of directly proportional spells are up my alley.