It's a pretty common question. I get it all the time from engineer/software/highly analytical students. My artsy/creative/intuitive students don't seem too concerned with the exact height to start their round out and flare, although they do have their own set of quirks.
I teach my folks to look down the runway and start their flare as the runway edges appear to start widening out significantly in their peripheral vision. Seems to work most times and helps with their alignment as well.
A wing span or so to begin the round out is a good rule of thumb, as long as they don't finish the round out a wing span above the runway. A foot above it is better.
If folks are looking at the altimeter to judge when to flare, I cover it up.
The only instrument that really matters in the pattern is the airspeed indicator, and even then only until you get the sight pictures down because I'm going to cover it up anyway.
It's a great learning experience to cover all the flight instruments and just fly the pattern by sight pictures and power settings. You'd be surprised at how well you can do.
Also, if the round out and flare is high, they're probably looking too close in front of the nose.
And we don't count 'em if the the center line is outside the mains.
