I’m surprised that it would affect mpg as much as you report. Are you certain of this? Have you repeated the measurements several times over different driving conditions?
Unless you live in an area where you get contaminated gasoline on occassion, I doubt the need for it at all. My 20 year old fuel injected Corolla w/200K has never even once had a dose of injector cleaner, and still gets the same mileage as it ever has. I think a better use for your money – if your objective is to have pristine injectors – is to replace the fuel filter once a year.