As already stated, this is a personal thing an depends on your strengths. Assuming that you want to ride on the slower side average of 20-25km/h (12.4-15.5 mph), running a 42x18 (assuming 23c tires), you'd need to average around 70rpm just to hold the low end speed of that range. Increasing the cadence to 85 RPM would get you into the upper echelon around 16mph but that would be hard to hold down for long periods of time. Competitive cyclists pedal at an average cadence of 80 to 105 RPM. None of this even takes into account watts.
I would measure the average cadence that you can sustain for long periods of time and your desired speed on flat ground and play with gearing from there.
https://www.bikecalc.com/speed_at_cadence
