I made up a new batch of "4 dKH" water yesterday, using the 6 gram number and it worked ok, based on my KH test kit agreeing that I had 4 dKH water. So, I think that is the number to use. I found that it is easier to do this by weighing the baking soda and mixing with a liter of distilled water, then diluting that water down to 4 dKH, than it is to try to do it by relying on the KH test kit and adjusting until you reach 4 dKH. I did it by using 6 grams of bicarbonate of soda in one liter of distilled water (200 dKH), mixing 10 ml of that with 90 ml of distilled water to get 100 ml of 20 dKH water, then mixing that with 400 ml of distilled water to get 500 ml of 4 dKH water, which is the size of my bottle. Pretty straight forward. My test kit said I had just a bit less than 4 dKH, but that could be either my error in mixing or an error in the kit. It is close enough for my purposes anyway.