A measurement for mixing the two parts for larger volumes is quite simple if you can measure in milliliters.  Many pharmaceutical measuring devices are available in ml increments (such as lids that come with cough syrup).  Since a 2 ounce bottle equals 60 milliliters and treats 60 gallons, it would take 5 ml of Part A and 5 ml of Part B to treat 5 gallons of water.  Other volumes would be equivalent (10 ml of each for 10 gallons, etc.).