We're going through the exact same math this morning. (The Ag Extension suggested going ahead with antibiotics although we cannot confirm a bacterial infection. I'm skeptical because of fears of overmedication and bacterial resistance, but we'll try it.)
There is a slight error in your calculations. (As an aside, I'm a chemistry professor and use unit conversions frequently).
We have a 280 g bag of Tetraoxy HCA-280. The directions on the front of the bag indicate that the entire 280 g bag can make 128 gallons of 800 mg/gallon oxytetracycline HCl, or 512 gallons of 200 mg/gallon oxytetracycline HCl.
We will split the difference and follow the rest of these calculations for a 400 mg/gallon concentration, as you have done.
For 400 mg/gallon:
280 g powder / 256 gallons = 1.09375 g powder/gallon (I'm with you so far)
Now, you convert grams directly to tsp. Good thought, but where did you get the density of the powder? More specifically, you seem to have used a density of 1 g/mL... but that is the density of water (at 4oC). Is that the density of the tetroxy HCA-280 powder? So, I'm at home without a precision balance. Rather than dragging the antibiotics in to work to measure mass, I measured the volume of the powder. As best I can measure, the 280 g of tetroxy HCA-280 has a volume of 1.667cups of powder. So, now we can find the density. (Density of the dry powder is 168 g / cup or 3.5 g /tsp).
So, to make the 400 mg/gallon oxytetracycline HCL solution, we need:
1.09375 g powder/gallon of water = 0.068359 g powder / cup of water = 0.020 tsp powder / cup of water.
Ok, that's fine. Now how to measure 1/50th of a teaspoon at home?
This ratio works out to a little over 1/4th teaspoon of powder per gallon of water for 400 mg/gallon concentration (actually 0.31 tsp/gallon).
I guess I'll mix some up now and see if our bird starts getting any better.
As an aside, does anyone know how I dispose of this stuff? We're on a septic, so I don't want to put it down the drain. I can pour it on the ground, but then I clearly am contributing to resistant bacteria. I can let it evaporate dry and then burn it...but that seems like a lot of effort. Thoughts?