I'm using a drop checker with 5 KH of distilled water in the cheapo red sea unit and man it's hard to distinguish the shade of green. It could be 6.7 or 6.4 and I wouldn't really be able to tell. I guess that is why I'm using 5 KH, as long as it's of some shade of green with no blue tint I should be above 30ppm CO2.If you want to know how much CO2 is in your water you can't do "roughly" measurements of pH, especially, and KH. You need to know the pH within +/-0.1 and the KH within +/-0.5dKH. That means either good titration test kits or a pH meter and a good KH test kit.
Unfortunately, you still won't know how much CO2 is in the water unless you use a drop checker with known KH distilled or DI water in the bulb. Aquarium water typically doesn't meet the criteria of having only CO2 and carbonates affecting its pH and KH.