I have a package of the Corid powder 20%. It does say on the package: 1oz = 3-1/2 Tablespoons 1oz = 28.35g. I measured again with my plastic tablespoon and a half tablespoon and the Corid package is correct. . But my package does not give the teaspoon measurement per gallon as you have circled in your example all the instructions on my bag are in gallons, ounces, mg, g, and kg.
What would be interesting to know if the Duramycin measures the same. I do not have a package to check for myself. But if there is a way to find out for sure so that correct per tablespoon measurements can be given to help the forum. If the Duramycin weighs the same as the Corid then the per tablespoon measurement on the Duramycin should/could be 400mg dose = 7/8 Tbls (0.25oz) and 800mg dose = 1-3/4 Tbls (0.5oz) per gallon of water. Still blows my mind that the Duramycin is so wrong.