How to use a Heil Goldline GM-4 microphone with a 6400M

I would start with a FLAT response! Every Voice is different! I have a Spectrum AUDIO scope that displays the entire audio bandwidth with a peak hold function.. I got this for my Hi-Fi system to fine tune my Dolby Atmos system.. Once you have the peak hold function, then I adjust my Flex to compensate for what my scope has told me..  Naturally you can play with it on the air as well, asking folks how you sound and compensate for how they are hearing you.. Over the years I lean to things being Flat Response, but this is very subjective based on many factors..

This device does a decent job, BUT!!!! Be aware this comes with a special test microphone, most sold on eBay DO NOT have the microphone.. My test Setup is the Heil Microphone feeds a wide-band Preamplifier that feeds my Yamaha Stereo system tunes for Dolby Atmos. Then the microphone is placed in the center of my Speaker System, and I talk into the microphone to determine voice frequency response...  Not something everyone can do or will do, but again On Air testing can help... 
Many of my contacts have the ham telling me that I sound like I am using Collins Radio Gear, and are surprised when I tell them FLEX!!

https://www.ebay.com/itm/dod-sr-rta-real-time-analyzer-with-pink-noise-mic-/202971977880?item=202971977880&ViewItem=&nma=true&si=ZnmKMoK7BBkl4Fc7v3jUdHebyYE%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

Good Luck!!
Robert