I don't know why the vet would say only half the amount we usually give chickens, but I've always given them 9-10 cc/ml per gallon for 5 days. If they are dying, you must hit them hard. Amprolium is not an antibiotic so it won't harm them. The preventative in medicated feed is amprolium and the treatment for cocci is the same med at a higher dosage for treatment. It's just a thiamine blocker. If you dose too low and they're already dying, it won't do them any good, IMO.