If you want to be exact, go to a local pharmacy and ask for a tuberculin syringe. (no, they won't think you are a junkie if you explain why you want one.) These syringes are 1 mL, with .1 graduations on the cylinder.
Your maintenance dose for daily excel according to seachem instructions is .1 ml per gallon... so .15 mL for the 1.5 gallon jar, and .25 for the 2.5 gallon jar. This is what I use to dose my small tank, (have a 1g vase) and it works great, allows much more control. also works well with micros/other ferts.