Is it better to set health - damage to a variable to have the equation process just once? I'm still trying to understand pass-by-value vs pass-by-reference, and whether it even applies in situations like this. Obviously the gains are essentially non-existent in this exact case.

From tooling's perspective, it is a good practice. While the return type can be inferred, the parameter's could normally be anything which is safe to convert to a number:

multiply("24", true)

Gotta type those arguments.