THE ACFT CRASHED SHORTLY AFTER TAKEOFF IN VMC WX CONDITIONS WITH AN ARPT ELEVATION OF 5145 FT MSL AND TEMPERTAURE 86 DEGREES F. DENSITY ALT WAS 7468 FT. IN THE PILOTS OPERATING HANDBOOK THE TAKEOFF DISTANCE VS DENSITY ALT CHART GOES TO AMAX ALT OF 7000 FT. A NOTE SAYS 'EXTRAPOLATION OF THE CHART ABOVE 7000 FT DENSITY ALT IS INVALID'. AN FAA AVIATION SAFETY INSPECTOR STATED THAT A TAKEOFF SHOULD NOT BE ATTEMPTED IF THE TAKEOFF DISTANCE CANNOT BE DETERMINED BY THE CHARTS. AFTER THE ACCIDENT THE ENGINE WAS TEST RUN THE THE MIXTURE WAS LEANED USING THE PILOTS TECHNIQUE. IT WAS FOUND THAT 100 MORE RPM WAS OBTAINED WHEN THE MIXTURE WAS SET WITH THE MIXTURE CONTROL LEANED AN ADDITIONAL 1/2 INCH. THE PLT HAD USED RWY 15 WHICH IS 5400 FT LONG FOR TAKEOFF. TAKEOFF ROLL WAS LONGER THAN USUAL & THE ACFT WOULD NOT CLIMB ABOVE 200 FT. THE TERRAIN WAS RISING TO THE SOUTH SO THE PILOT MADE A FORCED LANDING IN A PLOWED FIELD ONE MILE SOUTH OF THE ARPT.