I’ve read that using an index card on top of record can be a good way to set it due to the parallel lines on the card. My question is what am looking at to get parallel? The bottom of tonearm, top, or the bottom of cartridge?
Armtube must be parallel to the record surface when the needle is in the vinyl groove. Most tonearms designed this way and a cartridge/stylus also will be in correct position if the tonearm tube is parallel to the record surface. This is very easy to adjust if your tonearm have VTA adjustment (on the fly or not).
So when your particular tonearm tube is parallel to the record your cartridge is not parallel ?
What is your cartridge then and what is the stylus profile on your cartridge? For some profiles a tiny difference in VTA is irrelevant (conical for example).
More important: Are you sure you can detect any difference when you change VTA slightly? Why you’re so concerned about it?
The tone arm is a carbon fiber/aluminum 9 inch pro-ject. It does appear to have a slight taper towards the headshell end of arm.
If this is your tonearm are you gonna say there is no recommendations regarding VTA setup in the manual from Pro-Ject ? Almost any toneam i know comes with the manual and recommendation how to set up VTA correctly.