What is the optimal voltage setting to use on my cannabis vape 1.8V...4.2V?

The budtender at the cannabis shop told me 2.4; I figured they knew, since they're the pros.
Mine preheats (10s), so I often do that first (you can still draw on it).

How much was your vape thingie, got a link?
- I'm in Ontario, Canada, and this is price at local dispensary. I've had one over 18 months and haven't noticed any performance or battery depletion yet; picked up a 2nd one about 6 mos ago.

I'd recommend lowest setting that produces enough vapor to your liking - higher settings can "burn" around the heating elements and clog throughput. I've found 2.0 to be appropriate, but different cartridges might work with different voltages more efficiently

Need more context. Is this a concentrate? Are you smoking flower? What kinda vape?

Its a thc-only vape liquid (greenish goldish brownish) in a little screwable glass vial and a mouthpiece
The vape is a rechargeable (USB-C) vape pen with a dial at the bottom that lets you adjust voltage (i think == heat energy) from 1.8...4.2V. You screw the vial into the vape pen and double press a button quickly which makes it start heating the vial contents and you vape
- Sounds like you have a random 510 thread atomizer with concentrate in it. And you have a stick battery with 2 voltages maybe more. Try the lower voltage if you can pull through while hitting it 1.8 is fine. It's never a good idea to blast at high power as it can be detremential to taste and atomizer life. Hope your oil is from a legit source.
- Those are oil carts. I would recommend the lowest temperature that still gives you a hit. Higher temperatures will burn through it faster and get you higher faster but the low temps will gives you a better taste generally, and will release less bad things into your lungs and cause less irritation. If it's a really cold day, or if the cart is clogged that's when you use the higher temp. I personally think dry herb vapes are alot better in many ways though.

As said, the lowest setting that gives you a good hit, every coil and oil combo is going to be a bit different. The trick is to keep air moving through the coil while it's on, preheating or not, and to hit it slowly. Don't try to breathe through it, it's not a bong. Sip on it, take short hits with a few seconds between them. The oil is pretty thick, it's very possible to starve the center of the ceramic heater of oil on extended or hard hits, that causes this crusty burned residue to build up and the coil stops hitting right.

Slow and steady, let the coil refill with oil between hits.

I'd advise playing around with the settings yourself, especially once you've taken into account other answers that can act as a benchmark.

For example, I don't have a vape where I can directly control the voltage, but I do have a Flowermate, which heats at three settings: blue, purple and red. The blue setting is quite mild and the vape is quite flavourful to the extent that I can readily tell the difference between different strains (I'm guessing that at this temp, the vapour is fairly heavy on terpenes).

In contrast, the red setting provides a much stronger hit that tastes much closer to smoke than the vapour produced by the blue setting. This makes sense, because I think the red setting is about as high temperature as you can go without risking burning.

If I had a vape where I could control the voltages, I'd be able to test it to find out the rough mapping of voltage to these temperature settings that I'm used to.