I can't math, I already posted the dosage link so follow the instructions there for the waterer size you have.
I would not fill up the entire waterer. Make like 1 gallon at a time, unless your flock is large enough that they empty out the waterer daily. Easier to manage the dosage that way...
Why would you remove the waterer? If you're treating for an active case of coccidiosis they need to drink the Corid water for 3-5 days, then a reduced dosage for 7-14 days.
https://www.backyardchickens.com/articles/treating-coccidiosis-with-corid-amprol-ampromed-the-correct-amprolium-dose.73341/