FlyingForFun,
I teach to wait until you think you're going to hit the runway
This may work, but only if your student has a cautious height perception. If they have no concept of their height agl then this is very likely to end them up in trouble (esp. if they then convert to a bigger type using the same technique) with a heavy landing or worse.
I'd go with the End of the Runway Vs. Peripheral horizon technique everyday - but it's the least exact science of anything we teach
GSS