Using the above formula, I get 10cal/minute, or 600 per hour. Looking at a ridewithgps ride, 40 miles of rolling hills in 2:45 that is almost exactly what I get. So, yea, that is the formula they use also.
I use the numbers not for absolute burn, but for relative amounts comparing other rides. Also, using the easy-in-my-head metric of 150cal/beer.....well, you get the idea.I like that better than Zwift's pizza slices metric.