Lowest point before starting G/A? I agree with previous poster - prior to transitioning to flare. Approx 20'
If you haven't been cleared to land by the time you cross the threshold you must perform a go-around. Or so I was taught.
That said, I remember once flying a Cessna on the stall warner for 300 metres down Lanseria's runway waiting for the jolly chap in the tower to stop wittering to his mate and clear me to touchdown.
