teaching emergency landings...How low do you go...

My first instructor liked to have me practice emergency landings over rural
Oregon. Although we observed basic airspace rules, we'd get down below
treetop levels, short final, maybe 100' or less before go around. Once, I
could see the startled expression on a farmer's face as he watched us. On a
couple of occasions, I thought we were actually going to land. Birds took
flight, etc. Subsequent instructors and examiners always called it "good"
well above that so we never got that low.

My question is, what would you say the lowest appropriate height AGL for
teaching student pilots about off-field emergency landings in, say, a C-152,
given the 500' rule? (We -definately- busted that in the case of the
farmer. The instructor even said so.)


-c
CP-ASEL-IA, CFI student