@Robert Take a look this thread-
It looks like 1 tablespoon/gallon would be correct but read thru the thread carefully and make your own judgement.
The rate for Captan 50 (50% concentration) is 1 1/2 tablespoons per gallon if you adjust this rate for Captan 80 (80% concentration) you get 0.94 tablespoons per gallon which is approximately the 1 tablespoons per gallon recommended in the thread.