I purchased a bag of Amoxicillin 10% powder. The directions are 1 tsp per gallon of water.
I was going to mix up a half cup and keep it in the fridge when I need it. Just not quite sure how much powder to use.
I would see what your smallest measuring spoon is, and try mixing an amount that uses that.
1 gallon uses 1 teaspoon
1/2 gallon uses 1/2 teaspoon
1/4 gallon = 1 quart, uses 1/4 teaspoon
1/8 gallon = 1 pint = 2 cups, uses 1/8 teaspoon
1/16 gallon = 1 cup, uses 1/16 teaspoon
In most sets of measuring spoons I have seen, the smallest are 1/4 teaspoon or occasionally 1/8 teaspoon.
If you are comfortable eyeballing half of a measuring spoon, you could use half of whatever your smallest one is.
That still does not get you down to 1/2 cup of water, but is much less than a gallon, and can use things you probably already have in your kitchen.
It says it's 10g Amoxicillin per 100g.
I'm not quite sure which thing is being measured here.
Is that the amount of Amoxicillin in 100g of mixed solution?
Or the amount of Amoxicillin in 100g of powder in the bag?
O the amount you are supposed to give per 100g of hen?
Regardless of how it is being measured, if you are planning to give the Amoxicillin to the hen by syringe, the real point is to get the right amount of powder into her, with whatever amount of water is appropriate.
So it might make sense to figure out how much powder is right for the hen for each dose, then decide what is a good amount to mix up from there.
Some "mix with water" powders are meant to be in all the water the animal will drink, others are meant to go in just enough water that you can measure it and get the animal to swallow that much. I don't know which version you have.