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Easiest thing to do is set up one of your service monitors as a signal generator on your desired frequency.

I prefer to use the Duplex Out into the receiver antenna jack on the back.

From the RX test menu, Set the output port to Duplex Out and then generate a signal at -20dBm or so if you are totally off tune. Set the first tone generator at 1.000 kHz at 3 kHz deviation and set the second tone generator to your PL frequency like 0.123 kHz and at 0.7 kHz deviation.

Unmute the receiver by pressing volume up or down on the front panel.

Raise or lower the signal level of the generator so you can hear noise in the speaker audio.

Adjust the Helicals with a straight screw driver on the front of the receiver for minimum noise on the test tone.

Lower the output of the signal generator for more noise in the speaker and repeat tuning.

You should be able to get the signal generator down to -117 to -120 to where no more improvement will occur.

The alternative is to remove the helical assembly and set up the spectrum analyzer and tracking generator of the service monitor and tune the filter that way. You will need mini UHF to BNC adapters.
But pretty much you tune the filter to be flat top for 3 to 4 MHz wide. You should see about -1 to -1.5dB of loss. You can either center pass band to your receive frequency or this method allows an offset tune where you put the pass band edge at your receive frequency and you can gain additional rejection at the transmit frequency.