Some quotes from the B777 FCTM -
As the airspeed approaches V1 during a balanced field length takeoff, the effort required to stop can approach the airplane maximum stopping capability. Therefore, the decision to stop must be made before V1.
(my emphasis)
And -
When the takeoff performance in the AFM is produced, it assumes an engine failure one second before V1.(again, my emphasis) In the runway limited situation, this means the airplane reaches a height of 35 feet at the end of the runway if the decision is to continue the takeoff.
Within reasonable limits, even if the engine failure occurs earlier than the assumed one second before V1, a decision to continue the takeoff will mean that the airplane is lower than the 35 feet at the end of the runway, but it is still flying. For example, if the engine fails 2 seconds before V1 and the decision is made to go, the airplane will reach a height of 15 to 20 feet at the end of the runway.
And, finally -
What's important to note here is that the majority of past RTO accidents were not engine failure events. Full takeoff power from all engines was available. With normal takeoff power, the airplane should easily reach a height of 150 feet over the end of the runway, and the pilot has the full lenght of the runway to stop the airplane if an air turnback is required.
The decision should be made when the event occurs & the RTO figures are based on that occuring one second
before V1. In terms of speed, one second will differ on each departure depending on the aircraft type, weight, etc. For my operation I'm with Chimbu chuckles & my hand comes off the thrust levers approximately 5 kts before V1.