HLG reference white should be implemented like how it is handeled for PQ but at a level of 75%

[erazortt]

BT.2408 sets the same reference white level for both PQ and HLG at 203 nits. This equals about 58% for PQ and around 75% for HLG. Currently HCFR does not take into account any the reference white level for HLG and sets the expected brightness to 100% which is much too high. The implementation should be just like for PQ, with just another level of 75%.

See Chapter 2.1 in here: https://www.itu.int/pub/R-REP-BT.2408-4-2021

Since the value of 203nits is given for both PQ and HLG (table 1 on page 6) we can assume that this value is exact. From this it follows that the value of 75% is approximated and should be exactly 0.749877365.