I have a Cadmium Sulfide photocell. If I shine light on it, the resistance decreases. If I remove all light sources, the resistance increases.

According to the specification sheet, if I remove all light sources the minimum resistance is 20 M Ohms.

The maximum resistance is left blank. I noticed that if I test the resistance with a multimeter, the resistance of the Photocell goes past 200 M Ohms (towards infinite resistance).

So... is it reasonable to conclude that the maximum resistance heads towards infinite resistance? How does one 'calculate' a maximum photocell resistance

 

You cannot calculate anything.
The rated maximum resistance is 20M or more.
They are not all the same. Good ones are much more than 20M (the manufacturer doesn't say how high) and poor but passable ones are "only" 20M ohms.