Sorry, Rick, I misread your post. The problem is, at 7.8 PH, the KH will not be 3.5, because the amount of CO2 in the water will be about 3 to 4 ppm, not 1.7 ppm. The method of using 1.0 PH drop to get 30 to 40 ppm of CO2 is based on the deaerated water having about 3 to 4 ppm of CO2 in it, which my testing showed it to have (but, that might not be true for someone at way above sea level). It is a fun experiment to try to establish what the ppm of CO2 is in your degassed tank water - I used RO, carbon filtered water from the grocery store. I added a tiny bit of bicarbonate to get a KH, then diluted that with more RO, carbon filtered water to get the KH down to around 2. I let the sample stand a few hours - I found that 4 hours was enough - measured KH and PH and used the chart to get the ppm of CO2. Several of these tests at varying KH gave me an average of about 4 ppm. The biggest problem is the difficulty of determining an accurate PH for each sample. Try it for some amusement! Mathematically, going down 1.0 in PH has to mean the CO2 increases by a factor of ten, so it all depends on the degassed ppm of CO2 for determining how much that is in ppm in the tank.