Sofas are about 18" high, plus about 24" to a persons eyes, making 42" to the centre of the TV, then take away half the height of the TV, measure to the floor. That will be the measurement to the bottom of the set. This will vary as to the size of the TV. Better to have a TV higher than lower.

This really depends on you,as your the one watching the tv its what is the best place to site it for you,you decide where you want your tv to be then fit the brackets accordingly watching for hidden pipes & cables.
Good luck Alex

I had the very same issue a long time ago.
Too high your always looking up.
Too low looks odd.
All I would say is....
Sit on your sofa or chair etv and pick a point you are comfortable with (Will depend on the size of your tv also).
Focus on the point and mark it.
This should then be the centre point of tv.... brackets are normally on the upper 3/4's part on the back of the tv. So allow for this.
But once mine was done... looked great and no neck issues.
Hope it helps