Expression for a driver

I’m trying to set up a driver to control the Fac of a Mixshader which is dependent on the Z location.

What type of expression do I put in to a Driver expression to convert the z location, which goes from 0-5.5, so that my output value is in the range of 0-1? Is there a generic way to replace the 0-5.5 so I can use the expression for different value ranges, as I’d like to be able to use this for multiple objects with different Z locations.