I just did this on Wed for my chicks, so here's the math I did, adjusted to your chicks' ages.
It says on the label that 100 turkeys drink 1 gallon of water/day for each week of age, and chickens half that. So the process for chickens:
200 chickens= 1 gallon/day for one week olds
200 chickens= 15 gal/day for 15 week olds, or 15/200= .075 gallons per 15 wk old chicken.
Mix 2 tbsp per gallon of water (double check this on the label, though, cuz I'm doing it from memory)
For smaller amounts:
1 tbsp per 1/2 gallon
1/2 tbsp per quart
The dosing is the same no matter what age, but you have to make fresh medicine every morning. You can't mix up a 5 gallon bucket and let them drink it until its gone (unless they drink it all in one day)
I also added electrolytes to the water. I had to eyedropper in the water because my chicks were so lethargic. They perked up in just a few hours, but as of last night, they all seemed healthy (knock on wood)!
