This is one of the times I find the UI helpful. Fire up pro, do the operation manually, open up the analysis tools history, copy the the function as a python snippet, paste that somewhere, copy out the correct string for the coordinate system, put that into your code.
Another option is if the script touches a feature layer with the desired coordinate system, the script can query out the coordinate system value, stash that into a variable and use the variable in the calculategeometry operation.