With both PQ and HLG, 100cd/m² (nits) is encoded at codevalue 512, not 255. This is very unintuitive but even the SDR range of whichever HDR10 implementation gets more bitdepth…I suspect the much larger bt2020 gamut ‘requiring’ this for there to be no visible banding. Even sRGB primaries can show banding in 8-bit sometimes…makes sense, but is very counter intuitive.