I just had this issue come up with the recent basement renovation design. How high to hang a television over a fireplace, It is a bit tricky to have optimal tv viewing but still keeping it aesthetically pleasing.
The optimal height to the centre of a tv for seated viewing is 42". In this space there is a sectional sofa with a bar area behind so there will be seated and standing viewing. We have a large 65" Tv mounted at 60" to the centre but we are using a tilted mounting bracket that will help. This is just an example of how it helps to have your space laid out so you can make these decisions on paper before you have the fireplace installed and Tv mounted.