Determining value of capacitor, in a opamp ampifier circuit
C3 is working against R4 to form a high pass filter, not a low pass.  You therefore want to adjust C3 relative to R4 for the lowest frequency of interest.

To see how capacitors work as low or high pass, think what happens when you replace them with shorts (high frequency) or open (low frequency).

In this case, the overall opamp stage gain is 1 when C3 is open.  When C3 is small compared to R4, the gain is (R3 + P1 + R4)/R4.