John T,
'…but you ought to give some consideration as to what safety fudge factors you might wish to apply in the real world use of the POH numbers…'
If one considers the take-off requirements contained in the AFM and follows the appropriate instructions and orders I believe the solution is already before us. The C208B AFM for example tabulates the T/O Distance (GRD ROLL FT and TOTAL FEET TO CLEAR 50FT OBS) for a paved, level, dry runway – zero wind. The notes express percentage factors for headwind, tailwind, dry grass runway, and use of the inertial separator. If the final figure is then factored 1.25 as prescribed in CAO 20.7.4 surely no further fudge factors should be required.