From my 12 years of installation experience, I have a general rule of thumb for you.  This is a good place to start but by no means is this is a STANDARD or a requirement, just a starting point, that works more times than not.

1. Measure the highest "sitting" eye level in the location of the television.  ie if the couch height is 36", a chair is 40" and a bar stool 50"... choose the 50 inch level.

2. Based off that 50 inch level or the level in your room, place a piece of painters tape at that level on the wall you desire.  This is now your baseline.

3. From the baseline, position the height of the screen, not the entire television height, at the 1/4 point.  ie if it is based on the 50" plasma, the television screen is roughly 24" in height, place 18" above the line and 6" below the line.

NOW, what we have thus far is 50" baseline+18" screen height+3" frame size (estimate)= 71" TOP of television height or around 5'11".

Here is where the rule needs adjustment...

If the highest chair is 36", the height of the tv will be (36+18+3=57) 57" high which also means the tv is likely only to be (3+24+3=30, 57-30=27) 27" from the floor.  If there is any furniture below the tv to hold the av equipment, obviously this wont work well, or this will be an awkward height.  Adjust the height of the tv to accomodate any furniture plus add an allowance above it for speakers, nic-nacs. Some lowboy styles of furniture will work with this height.

EVERY tv mounting is different, EVERY room is different... use personal judgement and make the best call.  This rule isn't fool proof but it is a good starting point... good luck

All work can be fixed or changed if you decided to change your mind. But is so much easier getting it right the first time.