I can understand you wanting to double check your vets instructions as to dosage (I double check everything myself) but have I missed you giving the exact product and description? (strengths and thus dosage can vary)
Here is info I have on one product>>>is this what you have?
http://ph.merial.com/poultry/product_amprol_12.asp
Product Information
AMPROL
12% Solution
(VRI-02-1644)
A drinkable solution containing 12% amprolium hydrochloride for the prevention and treatment of coccidiosis in poultry caused by Eimeria spp.
BENEFITS
Fast acting against major types of intestinal and caecal coccidiosis.
Reduces the sporulation of some oocysts and thus, their ability to infect other chickens.
Allows the development of an anticoccidial immunity.
No cross-resistance with other anticoccidial drugs due to its unique mode of action.
Well tolerated by chickens and has no negative effect on weight gain, egg production or hatchability.
No adverse reaction with other drugs commonly used in feed.
DOSAGE & ADMINISTRATION
Depending on severity, mix 1-2ml AMPROL
12% Solution per liter of drinking water for 5-7 days consecutively.
(please note the above product is a 12% solution)>if you can provide the manufacturer and product details then it will be possible to google the info.
if it were me I would simply follow your vets instruction as to dosage (altho I would halve it if you do not have a sufficient number of birds that will drink five gallons in one day)
Some general info re amprollium:
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/200800.htm&word=coccidiosis%2cmedication
(from the text on drugs and their use in treating coccidiosis):
Not all modes of action are well understood. Some of the better-defined and more common agents are described briefly below.
Amprolium is structurally similar to thiamine (vitamin B1) and is a competitive antagonist. Because rapidly dividing coccidia have a relatively high requirement for thiamine,
amprolium has a safety margin of ~8:1 when used at the highest recommended level in feed. Maximal effect occurs about day 3 of the life cycle of coccidia. Because amprolium has poor activity against some Eimeria spp , its spectrum has been extended by using it in mixtures, usually with the folic acid antagonists, particularly ethopabate and sulfaquinoxaline. "
http://www.sheepandgoat.com/articles/coccidtable205.pdf
(A review of Drugs Used to Treat or Control Coccidiosis in Livestock and Poultry)