With your calculations, why is it .23ppm nitrate per mL? I thought the
concentration was .23ppm for the entire 100ml mix. But, now that I look
at it more carefully I see that you are right.

Here's the way I had planned on mixing using the Chuck Gadd's
calculator. I would combine 100 ml of water with 3 tablespoons of PMDD
mix and dose for my 12 gallon tank. However, the concentration is too
great after reading your post. With the 3 tablespoons and 100ml of
water, it yields a concentration of 6.8ppm nitrate in KNO3. If I then
multiply by 100, I would get 680ppm, which is way too high. So perhaps
1/8 teaspoon of my mix with 100 ml is a better solution for my tank?
How does that seem?

On the front of the bag it says, "PMDD Pre-mix one pound, contains
equal parts: Plantex CSM + B, MgSO4+7H20, K2SO4, KNO3."

Thanks for the clarifications.