CliveL and JohnT - thanks both for your answers.
(JohnT - I was rather hoping you were lurking as I knew this one of your specialties!)
But could I pose a follow up question?
It is straightforward mathematics to determine the time and distance required to traverse both the 2nd and 4th segments, at a given weight - say at MTOW.
But for my own operations; (DHC8 300) with a typical 15km surveyed obstacle clear area, are my RTOW charts ensuring, based on the backroom aerodynamicists work that you mentioned, that I'll reach final takeoff speed/config (to commence the 4th segment) by the end of the 15km? Or by any particular point?