Short answer: azimuth should be measured at the stylus to LP interface.
Ideally, you'd use something to measure channel crosstalk while playing a test record -- something like the Fozgometer, but even that method seems a bit controversial around here.
Many will place a thin mirror on the platter and rotate the cartidge until the stylus is perfectly aligned (straight up and down) with its mirror image. It's important that the mirror height is at the same level as the LP surface.
One can also get an analog to digital converter with USB output to connect to a computer. There are free softwares (sometimes included with the device) that can measure channel output when playing a test record.
I personally fine tune it by ear after using the mirror method. I simply adjust it until the sound no longer improves while playing mono records.
Ideally, you'd use something to measure channel crosstalk while playing a test record -- something like the Fozgometer, but even that method seems a bit controversial around here.
Many will place a thin mirror on the platter and rotate the cartidge until the stylus is perfectly aligned (straight up and down) with its mirror image. It's important that the mirror height is at the same level as the LP surface.
One can also get an analog to digital converter with USB output to connect to a computer. There are free softwares (sometimes included with the device) that can measure channel output when playing a test record.
I personally fine tune it by ear after using the mirror method. I simply adjust it until the sound no longer improves while playing mono records.