A question of Azimuth setting up turntable

In process of setting up my VPI Prime table and am questioning myself of whether or not I'm setting azimuth correctly.  All else is setup perfectly VTA, SRA, VTA etc. While setting up azimuth where should the measurement be taken? At the headshell or at the cartridge? While setting up my Ortophone 2m Black using a Azimuth block I checked Azimuth at the bottom of the cartridge while lowered on the platter which showed perfectly horizontal but at the headshell it was off by a fair margin. Nothing outrageous but it was off. Am I missing something or? Cartridge was installed at dealer but not fine tuned. So where does the audio community check and adjust their Azimuth? In my view azimuth at the cartridge should be preferred but I could be mistaken. Please correct me if I'm wrong. What is the correct or preferred way to set azimuth?