...at what point on the take off run did the Pitot start to register the headwind,...
The pitot registers airspeed at precisely the same point whether one has a headwind, tailwind, or no wind.
The airspeed indicator is registering the pressure of air entering the pitot tube, compared to still air (ambient or static air pressure). If one has a headwind on takeoff, one doesn't roll as far before the airspeed begins to register, but the value of air pressure into the pitot tube hasn't changed...the airspeed begins to register at the same speed of airflow regardless of the wind on takeoff.
If one has a tailwind during takeoff, one will roll farther, but the airspeed will begin to register at same ram air value that it always registers. The airspeed indicator isn't registering a headwind or tailwind...just airspeed. Once you have rolled fast enough to have enough airspeed, the airspeed indicator will begin to indicate, period. If one normally sees the airspeed indicator begin to rise at 40 knots, then one can sit on the end of the runway with a 40 knot headwind, and see the airspeed indication begin to rise. One can sit on the end of the runway with a 60 knot headwind, the aircraft sitting perfectly still, and see a 60 knot indication on the airspeed indicator.
I've done vertical and near vertical approaches to landing in airplanes such as Cessna 150's, in a strong wind, with no significant ground speed, but ample airspeed. The airspeed indicator doesn't care how fast the airplane is going, only how fast the air is moving past the pitot tube.