Substance Designer Community Wiki
FX-Maps
Pixel Processor
A formula works very similar to a normal substance graph except all actions are done through mathematics.
I would suggest when learning them to try and keep them simple originally such as if x = water parameter and y = intensity parameter you could use the value of water to adjust your colour intensity (saturation) on a curve that you control in the formula.
Remember that
Whilst there are hundreds of Curves these are just going to be some of the more common ones
Likely the most common curve is the quadratic equation y = ax^2 + bx + c it is used for making parabolas
http://werwackfx.com/index.php/graphictools/graphicsarticles/graphics-articles/64-functionsandcurves