Always start with the tonearm level and adjust +/- from there as needed AFTER the cartridge has run in for 50 hours or so.
However a level tonearm is irrelevant. You really want a level top of cartridge headshell. Tonearms are hard to sight in.
My method is to put a bubble level on top of the headshell when the cartridge is resting on a non moving LP. Of course the platter must also be level and you need to adjust the tracking force to temporarily compensate for the level weight. Then adjust the tonearm height. Document the settings for a level cartridge and adjust from there.