how much bac water do you add? (why there isn't one right answer)
Extremely common question, always asked like there's a correct number. There isn't, it's a choice with trade offs.
More water: more dilute, so a given amount takes up more volume and lands further along the syringe. Easier to measure precisely, more forgiving of small errors.
Less water: more concentrated, smaller draws, the vial lasts more punctures before it's empty.
Constraints: the vial has a physical capacity. Very small draws are hard to measure, a fraction of a unit isn't a real measurement on a standard syringe. And more punctures into one vial is more contamination opportunities over its life.
So the sensible framing is: pick a volume that puts your typical draw somewhere legible on the syringe, given the vial size and how long you expect it to last.
What makes it feel harder than it is: people search for the number instead of the reasoning, find a number that was right for someone else's vial and draw, and inherit an error.