What is the best method to time your round out or flare in Cessna 172?
The usual cause for flaring too high is that the pilot is looking at the runway in front of the propeller instead of down at the far end of the runway.

Next time you're up with your instructor, have him do the landing, and look at the far end of the runway while using your peripheral vision to judge your height above the surface. 

On the next lap, you fly the airplane, and try to get the same sight picture, both in front of you, and in your peripheral vision.