If I’m drinking two gallons of water per day, how much extra salt do I need to take on top of the 1/2 tsp in AM and 1/2 tsp in PM?

Extra water and salt.

Salt: A general guide for a well hydrated person is 1/4 tsp per quart of water. Adjust slow (listen to your body) and work up to an amount that works for you body. (Only if you are not on any medications, or have other medical conditions.)