Old Smoke,
I'm a flight Engineer.
I did have the rather unpleasent experiance of sitting behind a pilot conducting a take off on a 3*-500 empty where he didn't prevent it until about 10' RA and then decided to put it back on the tarmac for a while!!!!!
When the RTOW at max assumed is almost twice the aircraft actual TOW I would be very wary of using speeds for temp.