Alot of sources I have come across say a good place to start is a pint glass, just to get an idea of the overall taste. Then scale up if it comes out like you were anticipating if not you haven't wasted anything. You can also get a good idea of ratio for the larger batch. Try to work in 10 oz so you can have 1:9 or 3:7 or 1:1 and so on, then that scales whether you use ounces, gallons, pounds or bootfuls.