Looks like one 4 oz packet to 2 gallons of water to make a stock solution, then mix one ounce of that stock solution per gallon of water.
I don't recommend mixing up a stock solution, (it says make a new one every day!). Like I said, thiamine breaks down quickly. So I would add it directly to the water.
I will try some maths here: So thats 2 oz vitamins per gal water for a stock solution, that makes 128 oz of stock solution, one of those oz per gallon, 2/128 = 0.015 oz of your vitamin packet per gallon of water. If it weighs the same as salt (not an unreasonable assumption), my kitchen scale tells me that you need 1/16 tsp, affectionately known as "a pinch" per gallon of water.