Easier way to measure to make drop checker solution without scales or fancy equipment
You'll need 2 one gallon containers of distilled water. A 12 ounce empty rinsed soda can and a teaspoon.
-Add teaspoon of baking soda to gallon of distilled water. Shake vigorously. This is your 40dkh solution.
-Take the unopened bottle of distilled water, and pour it into the soda can until the soda can is full to the opening. Discard this water.
-Fill the soda can with your 40dkh solution until it is full to the opening. Pour the soda can contents into your bottle of distilled water. This should be 4dkh now.
Now the reasoning behind this. I measured a teaspoon of baking soda several times, and it's about 4.8 grams. Ideally we want 4.5 grams per gallon, this will be 6% over what we want.
A filled soda can contains 362 ml of liquid. Ideally we want 378 ml, but it's close enough, and about 4% under what we want, so the 6% over and 4% under should cancel each other out to within 2%. 362 ml of liquid diluted to 3.78 liters gives us about a 10:1 ratio, so 4dkh.