Coach:
My brother has a Ph.D in physics from Cal Tech.
I'll run this question by him.
My guess is that using a differential calculus approach to the question that takes into account the deceleration will give a more accurate result, but that it will be close enough to what both you and I arrived at to make it not worth the effort.
I'll let you know. He does this kind of stuff while brushing his teeth.
It's a pretty interesting question.