I wouldn't really recommend that method. The correct setting of an adjustable neutralization cap is the one that creates minimum feedthrough of the driving signal from the grid(s) to the plate(s) of the PA tubes, and this can only be measured or adjusted with the plate and screen voltage supplies turned OFF. On most tube-PA transmitters having any sort of adjustable neutralization, there is a switch to disable the HV and screen voltage to the final tube(s), which allows the user to adjust the neutralizing capacitor safely while the rig is in the TRANSMIT mode. Of course, this setup produces very, very little power output from the transmitter (ideally, it would be ZERO), too low to measure on a Wattmeter, but definitely possible to see on a Microwattmeter such as a Boonton 42 or 32, or many fine products from HP and others. Or, you can do what I've done many times, which is to attach a large 50 Ohm attenuator to the transmitter output connector and run the attenuated signal to an HF receiver tuned to the frequency of adjustment. With the PA plate and screen voltages shut off (but the plate tank must still be connected to the tubes!), place the rig in transmit and peak the GRID tune (or DRIVER tune, whatever it's called) for maximum signal received. Then adjust the NEUTRALIZATION capacitor for minimum signal. You should see a smooth dip on this adjustment. Once the neutralization adjustment is dipped for lowest possible output (this is the feedthrough signal that is leaked from the grid to the plate of the final stage), you're done. Sometimes it's best to see if this adjustment holds with changes in frequency, by checking on a lower band like 3.5 MHz and also on a higher band like 28 MHz. After neutralizing, turn the plate and screen voltages back ON, and you're good to go. This is a safe and sure method, and really the only reasonable method, for making this adjustment.