Fill a glass with your tank water and let it sit out for several days to allow full off-gassing of the CO2. This can be accelerated by putting an aerator in the glass. Measure the pH daily and, once it stabilizes, you'll have the starting baseline. Subtract your fully gassed 6.6 pH reading from that degassed pH.
Drop checkers are rough guides and highly dependent upon your being certain that the solution is 4 dKH (I run mine using a 5 dGH solution). They should be placed in the tank where you would expect the least amount of CO2 to reach. They are also closely tied to the pH KH calculations (charts also reflect this).
The problem with all of this is that you need to make sure that your pH and KH readings are as accurate as possible and it is not really possible to get high precision with the types of testing approaches that we use in our hobby, although they can be improved significantly through calibration, test modifications and moderately higher-priced offerings. Depends upon how deeply you want to go in terms of accuracy.
Simplest is to try to target a 1-1.4 pH drop as described above.