An eye dropper drop is about 1/20th of a ML. Knowing that, here's what i'd suggest.
Get your extract and a new, clean eyedropper and a beer that's similar in style to what you want to brew. Pour yourself a single 4 ounce serving (measure it) and add a drop of the extract to it. Taste it. Need more extract? Pour another measured 4 ounce sample then taste. More? Less? Keep pouring measured amounts until you know the right level of drops to 4 ounce pours.
A standard beer is 12 ounces. Once you have the drops measured out for a 4 ounce pour, multiply it by 3 and sample that. It should taste pretty much dialed in by this time.
Now, just figure out how many drops you need for your size batch. Let's say your 12 ounce beer requires 2 drops of extract and you have a 5 gallon batch. 5 gallons = 640 Fl. Oz. or a little over 53 bottles. If you know you need two drops per bottle, that's 53 x 2 = 106 drops per 5 gallon batch. If 20 drops = 1 ml, then 106 divided by 20 = 5.3 ml per 5 gallon batch.
You'll have to plug your own numbers to replace those values to figure out how much you'll need for your batch size. Don't forget to account for the trub loss and angel's share.
