Yes, you're right. It does give you a good approximation. I'd be willing to bet the true distance is within 7 or 8 ft of the calculated number. The gear shifting is usually not that significant on short distances like this, but the launch will play a major role in this distance. If you get a true AWD launch, with a little wheelspin and the car leaps forward without the engine bogging down, then the distance will be slightly longer (even though 60 comes up quicker) than your estimate. But if you do a street launch with a cluch slip and the engine bogs just a little, the distance will probably be slightly shorter. This can get very complicated.
What was the purpose of figuring out the distance in the first place? I don't see the point of a graph with only 2 sets of data.