Thank you , @Mark+P. Since writing the above, I also found the same path in the Registry, but …
… that key points to the program folder, the place where it’s installed; “Settings_Directory” points to the data folder, aka “My Paratext8 Projects”.
The full path to the key:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Paratext\8\Settings_Directory
But is this the actual key that PT leans on when it wants to know where the data folder is?