The last post from ToniLee is exactly correct and some of the other posts had incorrect information. I'm a pharmacist and I just calculated it out, (after I measured the exact amount of powder in one package of Duramycin-10, 6.5oz, just to make sure.)
One package has 13 tbsp in it and you need roughly (rounding up for simplicity reasons) 1 tablespoon of Duramycin-10 per gallon of water. That breaks down to 3/4 teaspoon per quart of drinking water. The solution is stable for only 24 hours and new solution must be mixed up every day. Treatment is NOT for 3 days (as posted earlier in this thread) but rather for a minimum of 7 days or you will get resistance.
And trust me, you do NOT want to develop resistance to this antibiotic or you will have to move to Tylan, which is much more expensive and harder to obtain. It's all to easy to get resistance to CRD in large flocks (50+ birds), as they keep spreading it from one to another. So just as soon as you get done isolating and treating some of your chickens and think you have it erradicated, you lose another chicken to it. Pretty soon, the drug doesn't seem to be working and voila, you have a resistant strain of CRD to that antibiotic. (I just had that happen, and with Tylan of all things.) Don't let it happen to you or your losses could be very high.
So in summary...
1 package of Duramycin-10 (tetracycline HCl) = 6.5 oz
1 package = 13 tablespoons
800mg/gal dosage: 1 tbsp per gallon
800mg/gal dosage: 3/4 tsp per gallon
Hope that helps!