TV mounting in Oakville can mean a century home near Lakeshore Road or a newer house out in Joshua Creek, and the walls are nothing alike. Old plaster over wood lath wants a light touch and careful piloting. Modern drywall on standard framing is quick by comparison.
Either way I find the framing first, then set the height against your seating rather than the mantel. Cables are hidden inside the wall when the construction allows it. The quote is free, the price is agreed up front and the room goes back the way it was.