If a sample of water has a pH of A right after it is taken from the tank, and that pH increases to B after you wait long enough for it to be at equilibrium with the atmosphere ( assuming you didn't allow a significant amount of the sample to evaporate), and you have determined that the ppm of CO2 at equilibrium with the air is C, the ppm of CO2 in the sample when it had a pH of A is D:
D = C x 10 exp(B - A)
ppm of CO2 in a sample equals known ppm of water at equilibrium with the atmosphere times ten raised to the power of the pH at equilibrium with the air minus the pH of the sample when you first removed it from the tank.