You have to make a few assumptions to be able to do that math, like Stonehenge and Alicorn's projectiles carrying the same weight and Stonehenge's projectiles fall to the ground at its range. We have the fact about Stonehenge that it can fire something at up to 6 km/s but not a thing about the projectile's mass. Also, we can't tell whether the projectiles continue to travel upward upon arriving at the range or fall to the ground. The difference in power output between these two cases should be massive to say the least. Not to mention Alicorn's range was drawn around as wide as Stonehenge's. I don't know if Stonehenge's range can still be said to be 1200 km at this point.
As for Alicorn, in SP02, its projectiles need at least 10 seconds to travel from somewhere north of Tyler Island to Anchorhead Bay. We also know the nuclear shell's weight without the railgun shell casing and that the Alicorn is capable of bombarding Oured with it from 5000 km away at an angle below 45 degrees. The missing ingredients here are the weight of the casing, and the angle but some people can find it out in-game if given time.
If we just know how much weight Stonehenge's projectile has to fire at 6 km/s, we can work out much closer to finding its power output.