I usually do some rough math but test in small amounts before settling on what I'd consider a comfortable dose for one can of seltzer. The seltzer is zero calories so you can always drink another.

Let's say the flower is 15% potent. There are 28.4 grams in an ounce. That means the amount of THC in that ounce is 28400mg * .15 = 4260mg.

You'd soak that ounce in Everclear which is 180+ proof. They say it's a 100% extraction rate. I usually use two cups of Everclear, but I don't get both cups fully out.

Assuming that you end up with 1 cup (237ml) of tincture that contains 4260mg, then the absolute max mg per ml of tincture is 4260 / 237 = 17.9mg of THC per ml of tincture.

Then if you evaporated off half the alcohol over 3-4 weeks, you'd end up with (max of) 17.9mg of THC per .5 ml of tincture.

In reality it's probably less than that because some of the THC probably remains with the plant matter that you throw out, but it's a good place to start to figure out what the absolute max would be.

I usually do the above and just from personal experience I think .5ml is between 10-15mg and one seltzer with .5mg is usually good for me. It kicks in quicker than other edibles, you can always have another one.

I try to keep it around .5ml per seltzer just so it doesn't taste as much like alcohol. The freezer extraction method for four hours is quicker and safer than heat extractions and it apparently results in less-awful tasting tincture. Evaporation also improves the potency to alcohol ratio which improves the taste.

Apparently you can let almost all the alcohol evaporate and then dilute the result with vegetable glycerin, but I haven't tried that as it seems like the result is super difficult to work with if too much alcohol evaporates. You do have to stir the tincture before using because you can visibly see the potency settle at the bottom.