The average stovebolt cooling system holds about 3 gallons or so. I'd suggest using around 1 cup of the acid. Like the high school chemistry teachers used to pound into our heads (back before the "education" system became "indoctrination") pre-mix the flush by trickling the acid into the water to avoid a nasty reaction. NEVER pour water into acid! Use a plastic container to mix the flush, never a metal one.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!