I got some nail polish remover bottles at Wal-Mart, removed the labels and marked them in 2oz gradations. The various types of these bottles come in various sizes that are exactly right for whatever your fuel capacity is and they are transparent plastic. If I'm doing less than a full fill I just add 2oz for each 5gal.
For motorcycle fuel tanks I use travel-size mouthwash bottles. You can always tuck a couple of them away somewhere on the bike.
I think you could get most if not all of the benefits at half the dose. I once removed my intake manifold and all valves were visibly oil-coated (and clean), presumably from the MMO at 4oz/10gal. There were also some threads some time back about double-dosing or more, there were no apparent ill effect but no apparent benefit either.