This is hopefully my final attempt at calculating this thing. Obviously, we can’t use near total fatalities, and I have no clue how to calculate ring based explosions. So what I can think to do is calculate the energy needed to move the storm at Pinkie Pie's house, then multiply that by the difference of the Rainbooms circumference versus the length of the storm. The logic is based on the idea that since the Rainboom ring was so massive, the amount of it hitting the storm was only a tiny fraction of it. I mean, honestly, it’s a storm, how big can it be compared to a blast ring expanding over a country? I think this image is a good illustration of what I'm trying to say. We can also see the blast ring affect the clouds when making direct contact with them in other scenes like this one, so the entirety of the blast ring clearly isn't limited in power to just what hit the Rock Farm.
First off: Methodology. My method for finding the ring size of roughly the same as before, but with one small difference: Rather than randomly assume a train speed (This thing is being physically dragged by ponies after all), I’ll find out how fast the ponies are running and use that for a speed. In this scene, they first appear at 0.80 seconds, and reach the other end of the screen at 2.69, making the time frame 1.89 seconds. I previously calculated stallions to be 1.6856396 meters tall. The height in this scene is 23 pixels and the screen is 1007 wide.
1.6856396 * (1007 / 23) / 1.89 = 39.048518 meters per second
It has just occurred to me that I have indirectly calced a running speed feat for the ponies, putting them in the Subsonic range instead of Peak Human
39.048518 * (3600 * 24) = 3373791.96 meters in radius
The clouds go all the way to the horizon, which is 20 km in radius by our standards, which means 40 km width. Circumference is Radius * 2 * pi. I’ve been informed by DontTalkDT that assuming standard radius is just fine even if the sky is filled with clouds as it won't affect visibility.
(3373791.96 * 2 * pi) / 40000 = 529.954002x
Speaking of storms, it’s time to find this one. Here’s the scene. The Sonic Rainboom makes itself known at 17.65 and ends at 24.48. The time frame is 6.83 seconds. I’m not 100% certain if these are cumulonimbus clouds. I know they go to the horizon at least. I think I’ll go with something smaller like nimbostratus, which according to the cloud calculations page, is 2 km thick minimum.
(20000^2 * pi * 2000) * 1.003 = 2.52081395e12 kilograms
2.52081395e12 * 0.5 * (40000 / 6.83)^2 = 4.32304119e19 joules or 10.3323164 gigatons
4.32304119e19 * 529.954002 = 5.47565244e12 joules or 5.47565244 teratons, or Small Country level+