So, if I mix the 10 ml with 339.4 ml of distilled or DI water I get 40 dKH water? If so, I can store that easily in a 500 ml bottle, then mix 50 ml of that with 450 ml of distilled or DI water to get 500 ml (another bottle) of 4 dKH water, and not have to store so much water. Did I calculate the 339.4 correctly?

This also lets me easily make 5.3 dKH water by mixing 66.3 ml of the 40 dKH water with 433.7 ml of distilled or DI water to get a drop checker to turn green at 40 ppm of CO2.

This is a big service to us hobby folks! And, it even standardizes our measurements of CO2 concentrations so our discussions of CO2 will make a lot more sense.