Juan, I was waiting for someone to throw that out there. I thought about going the "half as much as a 32 oz serving" route... Let's see, there are 128 oz in a gallon (unless you are talking about an Imperial gallon, which contains 160 Imperial ounces, of course), so that would be 1/8 of the amount needed for a gallon.
Or you could just do what I do and dump 3 heaping spoonfulls in and shake it up. If it needs more, then ad more. If it's too strong, I slug a couple of swigs and add some water.