DIY CO2 is not as "productive" as pressurized, and unless you have a pretty small tank, that .7 bbs is not much CO2. Do you have 4 dKH distilled or deionized water in the drop checker? How big is the aquarium?
Yes, the use of tank water makes the drop checker just a tank water pH indicator. You need to use the 4 dKH distilled or deionized water in it. You can get by ok by making your own 4 dKH water, using grocery story distilled water, and baking soda. Use your KH test kit while mixing, diluting, diluting again, starting over, etc. until you finally have 4 dKH water. It doesn't have to be exactly 4 dKH - 3.6 dKH or 4.4 dKH give just about as good a result.