On my plane if the radar altimeter crosses the threshold at 50ft, the landing gear crosses at 22', so dipping 20 feet low and I will hit the runway edge lights. Don't flare long or reduce power too late.
The 737 radio altimeter reads 0 at touch down - i.e. distance between my sole and ground. I would have thought the same applied over the approach lights with slight variance for different deck angles, perhaps?