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.
128x128audioguy85
millercarbon pretty much nailed it.
 I also start with dropping the tonearm a few millimeters and start from there the sound will be very 2D and "flat " sounding. When you get near the right height the sound will become more 3D  having depth to the sound.

I've watched Fremer on an Analog Planet video use a USB camera to set SRA , he got it to 93 degrees said it was close enough and would have to fine tune by ear anyways....I don't see the need of the camera then..just my $0.02.
I used a lively jazz recording without a lot of vocals.

Just take your time.

Scott 
Thank you guys for all your input! Certainly some very good techniques on display....now to find some time to do it lol. 
I use a sequence when aligning a cart for the first time.
1) Use a protractor to set ballpark overhang and zenith.
2) Use a VTA block to set the cartridge Upper surface  parallel from both the front and the side.
https://www.amazon.com/VTA-Azimuth-Gauge-Alignment-Block/dp/B0771Z3XMH/ref=sr_1_17_sspa?dchild=1&...=
3) Set the VTF
https://www.amazon.com/Riverstone-Audio-Record-Level-Turntable-Resolution/dp/B076DFZDS4/ref=sr_1_2?d...
4) Align overhang and zenith to protractor.  Check VTF again.
5) Sound test.  Crosstalk out of balance?  Dial in azimuth.  Sibilant or breakup on loud passage?  Realign zenith.    OK?  Then tweak VTF and listen for best. 
6) Adjust VTA/SRA.
7) Recheck everything.
8) Repeat.
9) Repeat
10) Repeat until it's right...
I do almost exactly what @wlutke does, except I use the VTA block to set VTA - as recommended by the cartridge manufacturer - and then check by sighting the actual cantilever, rather than using the cartridge body or pickup arm themselves as a reference.

Zenith is probably the most-overlooked aspect of phono cartridge alignment and one of the most critical, imo. I set it by using a mirrored gauge and aligning the cantilever itself.

It is true that the dynamic nature of LP playback means that things like VTA/SRA and VTF are constantly changing and with a pivoted arm, skating force and tracking error are also always changing. But getting proper static alignment helps limit the negative effects of the dynamic forces.