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Hi John,
One reason why the D850 could be failing to take a WB measure is over exposure of your white/grey reference.
The following process worked on my Z9:
1/ set my exposure params to cut down all ambient light (confirmed by taking a test shot with flash disabled), this is to make sure the measure isn’t biaised by ambient light.
2/ camera and strobe positioned roughly at the expected distance they will be used
3/ strobe power adjusted to create a roughly neutral exposure
4/ take a custom WB measurement: on the Z9 this fired the strobes and made sure the reference photo was strobe-lit, and the ambient diffuser taken into consideration.
Ideally, you’re using a slate or other neutral-grey reference, but in practice I’ve mostly used the sand… which isn’t exactly neutral and I often do a final WB adjustment in lightroom.
I hope this helps, let me know how you go?