KH test kits can be inaccurate too. Here is a way to calibrate that test kit, or make 4 dKH water, that has worked pretty well for at least a few people: https://www.plantedtank.net/forums/fe...tml#post800612
The fortunate thing is that you don't need very good accuracy on the 4 dKH standard. Since the ppm of CO2 you determine with a drop checker is directly proportional to that KH, a 25% error only makes a 25% error in ppm of CO2. But the drop checker isn't that accurate anyway. When it tells you you have 30 ppm, you may have anywhere from 20 to 45 ppm, depending on how good you can interpret the color of the drop checker liquid.