http://www.st-v-sw.net/STSWground.html
Most specifically, in regards to Starfleet's photon grenades. He works off of a 150 ton bomb to get in the range of a couple hundred gigajoules. However, I'm sure he didn't have this at the time:
http://meyerweb.com/eric/tools/gmap/hydesim.html
I was actually very excited when I learned that the calculator went down into the ton range. In any case, using the stated distance:
1,200 yards, that equals to .68 miles, to about where they were sitting. Going with this, I looked at the pressure from such an explosion would have at that distance. Unfortunately, hundreds of gigajoules are out, as at .25 PSI, the explosion would still have enough force to shatter windows. I am unfortunately, unsure what .1 PSI means, as the key doesn't go that low (at the bottom), so I decided to go safe and have the range of the weapon, so I went with 5.5 tons, which puts .1 PSI at .67 miles, so close enough that we'd probably see some small bits of dirt and such being pushed back (as we saw in the scene), but not enough to cause any real damage.Kirk: "An evaluation, Mr. Kelowitz. Where do you think they are?"
Kelowitz: "If I were them, I'd go to the high ground on the right. I'd make it 1,200 yards, azimuth 87. It's pretty close for one of these little jewels, Captain."
Kirk: "It'll be a lot closer to them. Stand clear."
Not bad for what amounts to a small grenade launcher with at most 2-3 miles worth of distance. I can't imagine what would happen to the first AT-AT that attempted to storm a Starfleet base.