Advice on setting VTA

I have set it before, but not exactly certain if I’m going about it the right way. I am generally setting it by eye, eyeballing the botton of cartridge body to get it as parallel as possible, sometimes using a 3x magnifier to assist.  I have also in the past used playing cards as a reference for some cartridges, so I have something to fall back on. Generally using the cards stacked at the tonearm base, similar to using feeler gauges. 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? 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.