Algebra is our friend:
1 tsp x
------- = ------
100 gal 300 ml
to make it easy, convert all English measurements
to ml (1 tsp = 14.78 ml, 1 gallon = 3790 ml)
14.78 x
--------- = ------
379,000 300
solve for x:
379,000 x = 4434
so
x = 0.0116 ml
= 11.6 (ul) micoliters

Rule of thumb is that a small drop is 20 ul,
so you need to add..drumroll, please...

half a drop!

-coelacanth

all that for a half a drop???? wow... you deserve a drink for that....