The output signal of the Zmpt101b is an attenuated sinewave with an offset of 1/2Vcc (about 2.5V). To be able to get a good RMS reading from this you need to know the offset and the attenuation.
You change the attenuation with the potentiometer on the Zmpt101b. You want it as low as possible (large signal) but high enough that you get a full sinewave signal (not clipped) up to say 260Vrms. To tune that you need to connect an oscilloscope so you can see the signal.
Once you have set that attenuation you can measure the offset voltage on the output with a multimeter by not connecting anything to the input. That’s your off-sett calibration, you must substract the offset, hence the negative number.
Now you can calculate your attenuation, by comparing the Vrms reading from your ESPhome device with the reading from a multimeter. You tweak that number until you get the same reading.
You will be working with mains voltages, be extremely careful please, double check every move you make.
Edit:
If you do not have an oscilloscope at hand, set the multiplier to a high value (say 360), get the off-set as described above, then tune the attenuation with the potentiometer, comparing the readout of your ESPhome device with your multimeter. I’m assuming here that you are measuring a 240V mains line.
éxito