I'm not aware of a calculator that does what you're describing. Like you said, it would likely have too many variables to make it very exact anyway.
If you're going to use a refrigerator, the best way to control it's temperature is an external temperature control unit. It will shut off the refrigerator's compressor when the probe tells the unit that the correct temperature has been reached.
Ranco Digital Temperature Controller (Not Wired) | MoreBeer
I tape the probe directly to the outside of my bucket. That reading should be within a degree or two of the temperature you want. It's a fairly precise way to control the fermentation temperature.