It sounded like Dr. Mark told her their was non-pathogenic coccidia in the feed. I am certain that
@MaryJanet will confirm for us when she gets on tonight.
Now in theory, you could take my post and adjust your therapeutic dose just like you said. Subtract out the final dose from your initial therapeutic and medicate their water accordingly. I would rather not deal with that variable. If they are pretty sick and aren't eating you risk the therapeutic dose being too low.
The problem
@Blazinga has is getting older chicks for the same place the coccidia laden ones came from. Not knowing if they have coccidia or not, I would immediately treat like they did. There is no effective downside. Wipe out anything that might be there and instead of the final tapering dose, move them to medicated feed. Then I could rest easy.