1/3 tsp = approx 1mg of Levamisole, which is about right for 10 birds in their drinking water. Directions on the packet say in enough water that will be consumed over 6 hours. I have also read elsewhere that the birds are to be kept on it for 1-3 days but they failed to mention dosage.
This is the best recommended product for gape worm, the directions are on the packet. Use the stronger dose for infestation, not the lower dose for maintenance.
The 5 gms per 8 gallons is for about 1000 birds! I'm going from memory, as I'm treating my flocks now. I will doublecheck and amend if I got it wrong.