That one is real tough to dose, as 1 gram equals 1000 milligrams. If each packet contains 10 grams of medicine, 1/10 of a packet would contain 1 gram of medicine. As you need a 400-800 mg dose, you can only deal in approximates with a powdered medicine. I'm afraid the closest you may get is using 1/10 of a packet to a gal. of water to equal 1000 mg. Yes, a little more than recommended but I don't think this will be a problem as you have such a range of effective dosages. You can also use 1/20 of a packet to achieve a more middle of the range dose at 500 mg. Good luck trying to separate 1/20th of a packet! This appears to be packaged for large groups of animals and not for a small flock.
You could also call your local vet and see what they recommend as a safe dosage.