This page points out something interesting:

you can have only *some* components of "HDR" in any particular display, so for example display can be Wide Gamut bt2020 BUT SDR! And/or smaller than 4k ;)

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Additionally, the individual aspects of the HDR specifications can, and often are, used in isolation. For example, there is nothing to stop a standard gamut display (Rec709), with standard HD or even SD resolution, working with an HDR EOTF with high contrast range.

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It also points out that viewing environment even more important for both HDR schemas - PQ and HLG.

Interestingly it includes table where specific (integer!) signal levels correspond to specific luminance in nits.

I still can't fully wrap my mind around what exactly "nit" represent, but apparently in HDR you really grade relative to specific max/min luminance of your mastering display AND suitable HDR display/TV must do some tone mapping magic if your defined luminance (?) goes above level that monitor/tv can show.

There was equation on page about tonemapping

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Converting a linear RGB triple to a luminance value is easy:

L=0.2126R+0.7152G+0.0722B