Method for Modifying Numbers based on Attributes in a Game [duplicate]

A game that I'm working on in Unity has a diverse set of spells. The player character and NPCs all use an attributes which modify these spells in different ways. Our design is very fluid and the functions that modify these spells are continuously changing. As of now I've been hard coding the modifying function into the spell class itself while leaving different variables open for designers to modify it. Example for amount of missiles fired:

BaseQty + Strength * MissilesPerStrength


Leaving the BaseQty and MissilesPerStrength attribute open in the inspector for designers.

As our game balancing is changing so much I feel as if only leaving certain variables open isn't enough, as I'll have to go back and change the formula itself if required. I've thought about allowing designers to script the function itself, and for the spell class to get the number for (Example) missiles fired by interpreting the script, but I'm not sure how to go about doing this and in my mind it feels a little bit needlessly complicated for the small team that we are. It's not too much of a hassle to go into the class and change the formula, it just feels wrong not to have it separate.

I'm looking for critique and suggestions on my ideas if possible. Thanks for reading.