You might find this interesting
http://codeandlife.com/2012/07/03/bench ... pio-speed/
though not up to date and doesn't include pigpio
you might find piscope shows you some things too.
Also not that the R^2 value is pretty much zero, that if i remember correctly means you best fit line for teh intercept is pants.
Also you should show the error of the final value. Just to complet it.
Nice though and we do similar (though not with the pi) but with a modulator and demodulator, via a laser and use an 'scope to detect a pulse (Picoscope DSO here)
also through a fibre optic, and use similar for speed of sound in metal rods.
Though If I have time i'll have a play with your setup too
That'll be a while though.