first you try one single drop and test how much surface it can paint.from that you can do some calculations for the amount of paint.but if you have some paint that wasn´t used get a bottle or something to keep it for another paint job.
If ya using pure color you wont waste paint anyway, just tip it back into the paint bottle even if you have reduced it, the main thing i'd suggest if ya dont want to waste too much paint is to buy a larger assortment of pre-mixed colors that way you can always add a drop or two more if needed of that pure color or return unused amounts back to its bottle, when trying to match a color is generally when many will waste some paint if they miss the mark, or run out half way through because they didnt mix enough so if you are mixing your own colors mix a reasonable amount and you can always use it for other projects..GL
Personly I would not put reduced paint back. I have been told that a chemical reaction occurs when the paint is reduced and starts a shelf life clock. You may end up ruining an entire bottle just to save a few drops.