Leveling Phantom I - armtube or headshell

Go for the Headshell.

....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.

I don't understand, either. I thought they should be aligned as well, but they're not exactly. I thought perhaps my arm had some sort of tapered armtube.
Thanks for the help. I think I will contact Graham about an upgrade to the Phantom II, and the micropoise.