I keep coming back to the equation for ppm of CO2 vs KH and pH:
ppm = A*KH*10^(B-pH)

The ppm of CO2 in the tank water is proportional to the KH of the water, so any percent error in KH gives an equal percent error in ppm. If I think I have 4 dKH water, and I really have 3.5 dKH water, my ppm error will be 100* .5/4 in percent, or 12.5%. I think most of us hobby types would kill to get that great an accuracy on our CO2 concentration.

If my point isn't obvious, we are flogging a dead horse here! Or, to put it another way we are on the wrong road to redemption. KH measurement isn't our problem.

Our problem is the pH measurement. That problem is solved by using the membrane type KH reference pH probe that Tom has posted about. Since the average hobby person isn't too interested in investing in such a device, and doesn't really need that kind of accuracy anyway, the colored water drop checker is preferable. And, for that a test kit determined KH reference solution is just what the doctor ordered. Unless the KH test kit is off by a factor of 2 or thereabouts, that is all we need.

Those, like Tom, who are into more accurate measurements of CO2, with very rapid response, should be looking hard at the pH probe based "drop checker" method. For them, getting a very accurate KH reference solution is a worthwhile goal. What the heck would I do with knowledge that I have 32.4 ppm of CO2 and not 25 - 40 ppm?

I purchased a 100 gram scale accurate to +/- 0.1 gram, a one liter measuring flask, a 100 ml graduated cylinder, a new KH test kit, a calibration weight, 2 gallons of distilled water, and, lest we forget, a box of Arm and Hammer baking soda - fresh, unopened. I could have used my existing KH test kit, my remaining half gallon of distilled water, my old box of Arm and Hammer soda, and made a batch of 4 dKH water and gone back to my experimenting. (But, I admit, it is fun to play Chemist!)