Tom, the perfect way to get a genuinely neutral grey negative, is to use a Wallace Expo/Disc, period.
They may have a shortened name now, but the Wallace Expo/Disc is a calibrated scientificly designed and manufactured, semi transparent unit, that slips on your lens, like a filter, except it slides on instead of screwing.
When you receive one, you will get a calibration card showing the deviance from 18% (I think from memory). I have two sizes, 52mm and 72mm, which went onto all of my Nikkor lenses.
Having used a myriad of different gadgets to obtain a correct grey, this is the absolute best. You can also check shutter speeds and/or lens f stops, by using different exposures.
Bob Mitchell had a small grey card made out of laminex, designed to go on a key ring, it is very good, but not as good as the Expo/Disc.
I've had mine since the very early nineties and did quite good to very, very good colour balance on my personal printing.
I have the original Jobo Colorstar 1000, which is like comparing a sixties BMW motorcycle, to a naughties BMW motorcycle, they are the same, but completely different.
To get a perfect grey negative for lab use for both colour balance and density, you slide the disc over the filter threads, turn the camera on to "A" for automatic aperture and/or shutter speed, point it at the light source, (generally), then trip the shutter.
Even if the shutter speed was down to, say ½ a second and you had camera movement, it won't matter, you will still have a perfectly exposed neutral grey negative.
You then take that negative and print a dead neutral grey tone, on colour paper, this is reasonably hard, actually.
Bob Mitchell invented another colour system which was designed to eliminate that guesswork, allowing you to get a perfect grey tone on your colour paper, he called it the, "Colorbrator", note the American spelling.
Basically this is an extension of the Unicube colour system, which I believe he also invented.
I have all of these different systems, they all work to different degrees of colour accuracy, all are quite good and quite accurate, but the combination of the Wallace Expo/Disc in the field, combined the Bob Mitchell Colorbrator in the darkroom, is perfect!
You can literally switch from daylight balanced film to tungsten, painlessly in the darkroom, provided you have exposed at least one frame from each film and lighting set-up first.
Once you have set your Colorstar analyser to a genuine grey, it is so easy, just put the probe under, turn the lights out, the enlarger on, then change the enlarger dials until all the colours on the Colorstar go out. Bingo, you will have a genuine neutral grey, and I do mean neutral.