I'm not an expert on the matter but Apple went with 1.0 for full SDR brightness, so you have to query for the maximum display brightness and explicitly use RGB values > 1.0 for HDR. This particular approach might not be a good idea for TVs but this kind of mapping seems easy to reason about and just makes a lot of sense to me. And since this does limit brightness for non-HDR-aware apps and content people figured out they can cover the screen with a multiplying overlay to boost the brightness up to the max.