Try taking the landing and take off distances over a 50 ft obstacle and
doubling them. Anything at or under those distances I'd consider using the
short field techniques.

I just grabbed an old Archer II POH and ran my finger across the table I
come up with a rough guess short field take off over a 50 ft obstacle
distance of 1500 ft from a dry paved runway with 25 degrees of flaps at
standard conditions. Double that and you get 3000 ft. Under 3000 ft I'd
use the short field technique. Flaps up, same conditions takes 1850 feet.
Double that and you get 3700 ft.

Between 3700 and 3000 ft of runway, it would depend more on the
circumstances and conditions.

I'd apply the same rule of thumb to landings.

These numbers may sound way to conservative, but they give you a lot of
fundge factor on the safe side and I'd rather be safe than sorry. Most of
us don't fly factory fresh airplanes and most of us aren't test pilots.

Jim



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