....leveling the headshell sets the arm much lower than leveling the armtube. For a Phantom I, which should I use to get a level cartridge?
I don't quite understand, the headshell and armtube run parallel to each other. When one is 'level' the other one will be also.
If you wish to get a reference level then set the stylus on a stationary record and use a ruler to measure at a few points on the arm tube - adjust so measurements concur.
I think Wally Malewicz used to have little tool for this - but any small ruler should do.
You should also consider contacting Graham to see if the 'micropoise' level can be added to your arm. Personally, I wouldn't be without it.