I'd be guessing that the C3 is "advertising" the 1370nits to Windows and hence that is what you see. Who knows it may even be correct for a small window (but certainly not full screen, but that would be rare).
Most HDR videos are encoded for 1,000 nits, some 4,000, and the hardly anything outside test patters at 10,000.
I'd just set JRVR for 1,000. The C3 will still do it's own mapping if needed.