You could find out with a drop checker. Find what you are reading right now. Turn off the air pump and measure again.
If you know how hard your water is (can calculate how much buffering it's capable of), then you could measure the pH fluctuation and calculate it.
FWIW, I had some surface agitation to prevent a bio film and couldn't get my drop checker to read 30ppm unless I turned up so much CO2 that the aquarium looked like a sprite bottle.
I adjusted the outlet for less surface agitation and was able to significantly reduce the amount of CO2 released into the tank to get to my desired levels.