Ah I see where the confusion comes from. For me, HDR reference white is not peak nits but diffuse white (I used to call this paper white but I switched after reading BT.2408-3). Basically the output value of ssts(1.0). I do not consider the inverse EOTF (or the OOTF in the case of HLG) at this point. Instead, my only consideration is: where do I want this to land in absolute land and I can validate this by doing a straight InversePQ(value / 10000) without any further processing.
Here’s the quote from which I take my definition:
The reference level, HDR Reference White, is defined in this Report as the nominal signal level of a 100% reflectance white card. That is the signal level that would result from a 100% Lambertian reflector placed at the centre of interest within a scene under controlled lighting, commonly referred to as diffuse white. There may be brighter whites captured by the camera that are not at the centre of interest, and may therefore be brighter than the HDR Reference White.
Graphics White is defined within the scope of this Report as the equivalent in the graphics domain of a 100% reflectance white card: the signal level of a flat, white element without any specular highlights within a graphic element. It therefore has the same signal level as HDR Reference White, and graphics should be inserted based on this level.