The centre of your tv should be about 60" from the floor.

Ok can the mantel be right underneath about 57" from the floor?

Do you need a mantel, I think if I was putting a T.V on the fireplace wall I would forget the mantel altogether and the center of your T.V. should be at a height that is at eye level when you are sitting down . The 60' level is for comfort when mounted above mantel, .What kind of fireplace, if it is wood burning this is the last place you want to mount your T.V. if it is a phony one I would forget the mantel and hang like in the pic.

As low as possible and also depends on how far the sofa(s) are from the TV.

Thanks for the advice this is what I'm going for.