The mantel decides
Over a fireplace the mantel sets the lowest the TV can go, and that's usually well above eye level. The fix is not to give up but to measure: from 12 feet a TV whose middle is 30 inches above your eyes asks your neck for about 12 degrees. That's watchable. From 8 feet the same TV is 17 degrees, which is r/TVTooHigh territory.
A tilting mount angled down by the same number of degrees keeps the picture square to your eyes. A pull-down mount solves it properly.
Questions
How high should a TV be mounted?
The middle of the screen at your seated eye height, usually about 42 in (107 cm) from the floor. For a 55 inch TV that puts the bottom edge about 28 in up, for a 65 inch about 26 in.
How high is too high for a TV?
When your neck tips back more than about 15 degrees to see the middle of the screen. Under 10 degrees is comfortable for nightly watching.
Is eye level the centre or the bottom third?
Most guidance says centre at eye level. THX suggests your eyes line up with the lower third of the picture, which raises the middle by a sixth of the screen height. Eyeline does both.
What size TV for my distance?
Enough to fill 30 to 40 degrees of your view (SMPTE and THX). From 8 ft that's 59 to 80 in, from 10 ft 74 to 100 in, from 12 ft 89 to 120 in.
How high to mount a TV over a fireplace?
As low as the mantel allows, with a tilting mount angled down toward your seat. Check the neck angle: over 15 degrees for nightly viewing is hard on the neck.
Where do I drill?
If the mount holes are in the middle of the TV back, the bracket's middle goes at the centre height. If not, measure from the top edge of the TV to the holes and put that in; Eyeline gives the hole height.