Carl,
You are correct. The theory was if you are accelerating at a linear rate then by starting the speed trap 66' before and ending it 66' after you would get the exact speed at the moment the car crossed the finish line. But as we all know, acceleration is not linear, so that was a flawed concept.
The speed now is the last 66', but it would be interesting to know for fact (and I don't) how often the car is still accelerating and how often it is slowing during that part of the run. Because with all the drag/downforce a cylinder out will have the car slowing at the stripe. On a perfect run, a TF car picks up about 42 MPH down the last 340', it is not accelerating very hard even hitting cleanly on all eight. Maximum acceleration is well before three seconds according to the G-meter, the car is still picking up some speed but at a much diminished rate past the 660' mark
I would be willing to bet that the old style (132') trap would show slower speeds on average than what we have today.
Alan