One item all this math doesn't take into consideration is the fact that we time from a standing start. The airplane actually has to fly faster than 70 if you are going to get the high speed time at or below the calculated 70 MPH time.
This is true. Since we time from a standing start, the model's top speed is actually higher than the high speed score.
However, we never have any occasion to time the high speed from a flying start rather than a standing start, so when we talk about the high speed it is a given that it is from a standing start.