@coach723 is correct, trust me... I am dealing with a confirmed coccidiosis outbreak myself. If your flock has it you want to treat for severe and you want to treat quickly.
You can give up to 8x the Amprolium recommended dosage without adverse effect. I'm not recommending you to do that, but to illustrate that a little too much Amprolium is far better than too little.
Also, you'll want to stay away from any nutrient boosters like Sav-a-chick, molasses or nutridrench while you're treating for Coccidiosis. I know it's tempting because they look so sick and you're desperate to help but you're feeding the coccidia with them, not the chicken.
As
@coach723 mentioned, Amprolium is a Thiamine (vitamin B-1) blocker. The way it works is by mimicking thiamine which is what the Coccidia feed on. They eat the Corid instead of the real Thiamine and starve to death. If you put more real Thiamine in the chicken during treatment, you're giving the Coccidia food.
Keep your sick hen hydrated with clean, fresh Corid water, and give her easy to digest food like her regular pellets/crumbles and keep her warm, dry and under your observation.
Is there a vet that you can take a stool sample in to tomorrow? Do you know for sure that it's Coccidia? What are the symptoms you are witnessing?