Well, all that monkey business with the making and weighign is about out the window.

The 10ml 25,000ppm NIST certified reference samples will likely be sold through Greg Watson, still waiting to hear back.

This way you no longer even test KH at all, ever again.

No need.
Then you only measure pH with a meter/O2 membrane or with the drop checker.

3.489 liters of pure DI water added to the 10 mls will make about 3.5 liters of 4 KH solution.

If you can measure 3.5 liters or a a tiny hair shy(use the 10ml bottle the ref solution comes in to remove the water or to get an idea how much 10mls is), your accuracy is still 99.984% for each 1 ppm of KH.
That assumes the volume you add is within 10mls or so.

I do not think anyone will require more accuracy.
Buying a 500ml volumetric flask will allow precise measure of 3.500 liters of DI water(7 fill ups basically), and you can use it for other standard ref solutions you might want to make for NO3 etc. They also make good stoarge solution containers.

3.5 liters of ref solution will last about based on 2 week changes and 5mls per drop checker, 26 changes per year x 5mls= 130 mls.

3500mls/130mls = 26 + years of accurate CO2 measurements and no KH test kits.
Saves money, time, reduces work load, highly accurate, easy.

Cost: about 10-12$ for the ref sample, 10$ for a 500mls flask, whatever the going rate for pure water might be, and shipping.

Still, about 50X cheaper than all the KH test kits you'll go through and time you waste measuring.

Regards,
Tom Barr