VTOSS
Well despite my own view, i thought i'd stick my head in a book for ye.
This from Bob Taits VFR PPL Study guide
"TAKE-OFF SAFETY SPEED"
A take-off safety speed shall be established for each flap setting for which take-off distance information is to be provided. The take-off safety speed shall be an airspeed not less than 1.2 times the stalling speed and at which adequate control is available in the event of a sudden and complete engine failure during the climb following take-off
IIRC, both casa and manufacturers performance charts list this figure in the take off conditions box. (where there is no calculation provided). That being said, IIRC it is a certification req.