In his example it's 1m apart so I took 1/c s.
If it were 300km than it would be 300,000/c s. 300km is a fortunate distance because the approximation of the speedometer of light in a vacuum is 300,000 m/s making the math trivial, 300,000 / 300,000 = 1s.
At least as far as my peanut brain understands it.
