How do you make an effect that starts negative, improves in the middle, and then ends negative?

So I want to make a simulation where the purpose is keeping it in the middle, or slightly above the middle. If it’s too high, or too low, it effects multiple things negatively.

Hello Skavau,

in my mod 'Inflation / Deflation Simulation 1.0’ for Democracy 3 the player have to keep the inflation/deflation rate in the middle to avoid some negative events.

The mod can be found here:

Maybe you can get some useful hints from my mod.
If you use some code excerpts from my mod, it would be great to mention my mod in your credits.