Oh I didn't actually bother to look up Mach 1, just took it from earlier posts in the thread...
At any rate, your approach still doesn't make sense. You can't look at the end points, find the average and say absolutely anything about what happens in between unless it's a linear equation.
If you want an absolute boundary use Vf^2=2ad and get 840m/s (assuming he falls straight down under sea level gravity). Might as well pretend he's a spherical cow in a vacuum.
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