Just to belabor this a bit more: NaHCO3 will add H2O to its crystal structure if it is in a humid place, so when we weigh a bit of NaHCO3 the result is the total weight of both the NaHCO3 and the H2O. We can bake our NaHCO3 to drive off the H2O, but in doing that we are also converting some of the NaHCO3 to Na2CO3. The former is 71% CO3, but the latter is only 57% CO3. So, by baking we are losing accuracy, not gaining it. I don't see any possibility of making an accurate KH solution by weighing NaHCO3. And, for sure I don't see a way to equal the accuracy of a fresh KH test kit.