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Mervian -

sorry, 'config' is my short hand for the folder that opens when you press the
Preferences->Miscellaneous->Open configuration directory button
I suggest you copy the contents of that folder (...\Appdata\Roaming\calibre) to say C:\Calibre\ConfigData
The value you enter for CALIBRE_CONFIG_DIRECTORY is the path to location of the new config folder - eg
C:\Calibre\ConfigData
Here's a 'user friendly' utility for manipulating Windows environment variables
Rapid Environment Editorr
Make sure calibre is closed whilst doing this. Once everything is working I suggest you should delete the ...\Appdata\Roaming\calibre folder on each computer.
BR