I have an even easier way: I poke a hole about the size of a 16d nail in the foil and squirt some in the 5 gal can or right in the tank. I figure I can guess pretty good about a "double shot" or a little less. If in doubt, squirt again. If I'm really off and I only get 1 oz/5gal, that's better than nothing. If I'm really off and I get 3 oz, it won't hurt a thing.