Decision speed in the context of a takeoff should only mean one thing. It is the point in the takeoff where you switch from automatically rejecting the takeoff by closing the throttles to carrying out the engine failure drills anf continuing the flight , in the event of an engine failure. If the airplane has a positive rate of climb , is at blueline, and the landing gear is retracting than it will fly away, therefore that is my light twin V1.
I think it is important to note transport category methodology is once you pass V1 you go. There is no big mental debate about wether there is sufficent runway ahead when the engine fails, You go EVERYTIME. Granted the difference is transport aircraft are still on the ground at V1 while the light twin will allready be in the air but the concept is the same. There is ample evidence that under the stress of a late takeoff engine failure a preprogrammed response that does not require intellectualizing the situation and then deciding on a course of action is the safest way to go in virtually all circumstances. In the context of this discussion why should we expect a low houred light twin student to make decisions not required of even a gazillion hour 747 captain. BTW I teach my students to keep their hands on the throttle untill blue line is reached, they then use the throttle hand to select gear up and then return to the throttle quandrant and firmly grasp both prop levers. The drill is really simple. If your hand is on throttles and you get that sickening lurch sideways , close the throttles, if your hands are on the prop levers feather and fly.