How to place speakers at the optimum height

IME, it is more important that the speaker(tweeter) be aimed to your ear than at ear level.

I agree with Zman but... make sure you level the speakers and make sure they do not rock side to side or back and forth. You can also try toeing them in some. Ex. 15'' from the back wall outside corner 16'' and inside corner 14''(middle of speaker 15'') then take 1 or 2 laser pointers (stand behind the speakers and aim them (from the outside corners) toward the wall behind where you sit) and have the lines crisscross about 1 foot behind your head. You may need to try several attempts or adjustments, but maybe it'll work for you. Lots of luck.

You can also read John Atkinson's measurements in Stereophile, provided the speaker you're interested in has been reviewed there. He measures and discusses the vertical dispersion of speakers and usually gives you hints on optimum listening height (unless it's not critical).

You could also listen to the speakers while squatting (sorry!) and standing to try to focus in on the height that sounds best.
-Bob

Your nose is on the same plane/level as your ears ... use your nose as a guide ... it's easier to see than your ears

DISCLOSER .. I am not telling you where to put your nose only to use it as a guide or reference point relative to tweeter

Make sure that the lasers are off when you are adjusting them while seated in the listening position.
I also think that as far as floor standers go they average about 3 to 5 feet. It is absolutely true the further you are from the speaker the vertical dispersion becomes less critical but using an 8 foot cardas triangle your head should be at the apex of the equilateral triangle so toe in is important. Remember it is your system and your preference so just adjust to your taste starting with those principles. Many speakers including my own have excellent lateral dispersion and don't require a lot of toe in. You will see, or hear, as the case may be.