SweetFX Tonemap Defog

I would like to know if anyone could give some tips or better understanding about how defog works. Is there a way to y know how to "calculate" which values to put or is it just trying a thousand times?

For tonemap defog I use RGB Color values like this:

www.madsci.org/posts/archives/2001-08/999275799.Ph.r.html
or this:
www.rapidtables.com/web/color/RGB_Color.htm

so the values of defog all go up to 2.55 and RGB color values like this go to 255 so I just translate.
Start out with a low amount of defog and work your way up until the image is neutral.

Hopefully this helps you out.