The second method works, but you already have the drop checker to be able to get both the tank and the 4 dKH at the same ppm of CO2, so why not just use the drop checker?

The first doesn't work, because it takes a long time for water to reach CO2 equillibrium with the room air - at least two days, unless you are stirring up the water to get surface turbulence. Then, the amount of CO2 in the air in a room of a house has to vary considerably due to human activity, cooking, smoking, etc. So, I don't see that working for that reason either.

I did some testing using very low KH distilled water in the drop checker, and in a small container, where I stuck the drop checker. So, both the drop checker and the container had the same dKH of distilled water in them. It works for measuring low ppm of CO2 in the container of water. I was blowing thru a straw into the container to add CO2 to reach a higher level to check how long it took for the drop checker to reach equillibrium with the container water. You can do several interesting things that way.