They give you starting percentages in the description for each product. Usually starting around .5 %. This is one of the times it would be more convenient to work in metric. Liters and ml.
I just bought a couple of the small dosing syringes that are made for giving medicine to pets, etc. I got a 1ml thats in tenths so I can measure fractions of a ml and also a 10ml one.
I plan to start with a small sample, say 10cl which would be a little less than 3.5 oz. .5% of 10cl happens to be .5 ml. Taste that, see if it needs more or less.
Say .5 % is good. And I want to make 3 gallons. 3 gallons is 11.35 liters. 11.35 times .5% is .056 liters or about 1.89 oz. Apex sells a 2 oz size which I have been buying, figuring this.
Now by the time you buy vodka to dillute and then buy your flavoring, it will most likely end up being cheaper and more convenient to just buy White Claw or Truly. The companies producing these are fermenting sugar instead of using vodka, and are working in massive volumes - which is how they are able to turn a big profit.
I would do this to make up a flavor I can’t buy. For instance, they have one that says Margarita flavor. And of course, just to say I did it.