In general you don't want to do math on PQ code values. They're even more nonlinear than gamma, and weird things happen.
Linear scaling of nits isn't that bad a way to adapt to brighter ambient light conditions, since PQ itself is defined relative to a 10 nit ambient. It's certainly better than trying to scale or add to PQ code values.
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Ben Waggoner
Principal Video Specialist, Amazon Prime Video
My Compression Book
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