At that ratio, you would need only 1/32 of a teaspoon of whatever it is you have to add to two cups of the other stuff.  Since 1/32 of a teaspoon is not a practical measurement, you should consider a different approach.  If you have a lab balance you can get ratios in mass instead of volume and make an accurate measurement.  Another approach would be to make a bigger mixture - perhaps 1/8 teaspoon per ½-gallon.