Parallel? How do you set the VTA?

Use your computer word processor and print out a series of closely spaced lines (similar to an indexed card but more refined). Cut the paper to a manageable size and fold in the middle along one of the lines to create a 90 degree fold. Place the “L” shaped lined guide on the platter. Lower the tone arm onto the horizontal fold and align the head shell with the lines on the vertical plane. Since the cartridge is affixed to the head shell, you want to make sure the head shell is level. It does not matter if your arm tube is tapered or not. Once level, adjust by ear.

Happily my ear tells me when the setting is off, but as a base, how do you determine conclusively your tonearm is parallel?

Why should I want to? You said it yourself, this parameter is best adjusted by ear. Visual settings like "tonearm parallel" are mere approximations, someplace to begin the listening from.

With respect, a quest to "determine" the position of a mere approximation "conclusively" is nonsensical. I can level a cartridge or tonearm by eye in about 15 seconds. Then, whilst you're fussing with spirit levels, printed cards, magnifiers and other distractions, I'll have dialed in the optimal SRA by ear and be enjoying the music.

I set my arm initially with a bubble level on the headshell. If it sounds bloated, I raise the rear. If it sounds too shrill, I lower the rear. Level is usually pretty close to the final result though.
Also a good starting point for Azmith, although I use a Fozgometer for final adjustment, and then my ear.

Manitunc,
Was it you who posted a link to a very small bubble level a while ago in a different post? The one Jmcgrogan linked to is way too heavy at over 6 grams.

Doug,
This conversation reminds me of the discussion we had a while ago about setting the VTF for the Delos at 1.75 g. Yes, my ear has doubtless become more sophisticated and discerning after I got into analog a few years ago, and as I've upgraded to a more resolving equipment, but some of us just don't have the same acute hearing that you do. It makes me feel better knowing that at least I can start my adjustments with a base reading recommended by the designer. If I'm lost in messing with the VTA, VTF, and overhang using different protractors in search of the optimal sound, I'd be reassured to be able to go back to the base settings. Just for that it is worth for me to be able to set up my tonearm parallel to the record surface, just like it is to set the VTF to 1.75g for my cartridge. It might be non-sensical to a seasoned audiophile like yourself, but it makes sense to me, a relative novice compared to many here.

Actusreus, you start with a new deck of cards and insert as many as necessary on top of the record so they stack up and are close to the arm. A bit easier to see if you are parallel to the bottom of the arm when the distance between the top card and bottom of the arm is small. My eyes and distance judgment is not as good as some responding so this additional aid gives me a better estimation of parallel for the starting point. After that I adjust by ear as and if necessary.