Accurately measuring how much CO2 is in the aquarium is virtually impossible. First, the only accurate devices for directly measuring the CO2 cost way beyond what hobbyists can afford, around $2000. Second, the concentration of CO2 in the water varies all over the map depending on where in the tank you measure it. Right around the plants, with poor water circulation, you can have near zero, while up near the top of the tank, near where the CO2 enters the water, you can have 40 ppm. With good water circulation in the tank, the differences won't be quite as pronounced, but still enough so that one number won't characterize the amount in the tank.
Using a drop checker serves one very good function - it lets you get somewhere near the maximum allowable concentration of CO2, just so you have a good feel for what it looks like when you do, what kind of bubble rate it takes, how the fish behave, etc. Then you have to make final adjustments by watching the plants and fish. And, to make it even more difficult, the final adjustments aren't final. Once the plants grow a bit bigger and more dense you will very likely be too low on CO2 again, because the plants use a lot of the CO2, and the more plant mass you have, the more CO2 they use up, reducing the concentration in the water. But, that higher plant mass also blocks a lot of the water circulation in the tank, which makes the differences between the concentration at various points in the tank even higher. CO2 may always be a frustrating thing to try to do "right".