A television is a black rectangle, and everything around it has to be planned in relation to that rectangle rather than to the wall.
The first line our artisans strike on the drawing is the horizontal centre of the screen at seated eye level - measured from the seat height of the sofa that will face it, not from a standard. Everything else is then hung off that line: the bay opening, the sill of the console below, the height of the medallion above, the top of the side niches.
The second decision is margin. A screen bay cut tight to the television looks like a hole; a bay with a generous, even reveal all round makes the same screen look set into the wall. We size the reveal to your actual screen, and we allow for a larger one later - because televisions are replaced far more often than furniture is, and a bay sized to the exact model you own today is a wall unit with a short life.
That is the whole design method. The screen sets the geometry, and the carving, the lighting and the storage are composed around it.