Well, from what I've seen, most of the FW cals are WRONG for KH measure.
Usign an inaccurate test kit is not a good way to measure how accurate the KH is in DI water, that will get you at most +/- 4 to 8ppm.
parts per billion.
Do you need it that accurate?
No, but why not?
If you make a gallon of it, it's easier to account for small weight errors, also, cook the bakign soda for 1 hour at 300-400F, this will drive off the water and CO2 that's sometimes left.
This will give a better weight.
Alk is quite different in how you measure the KH, do you measure the HCO3? Or the CO3? I've found 4 different sites that all gave 4 different answers, some quite different.
This one I had confidence in.
They have good chemist that bother this site frequently, and Randy is a heck of nice guy and has many folks pouring over the info and updating things as needed.
Reef Central has 1000's of post per day, it's hard to get feed back as it's so intense.
So there are so many folks going after such chem issues.