I can't help you with the math Dan, but someone on this board can.

What's amazing to me is the size of the cloud that produces the flash.

Remember they are traveling at 160,000 mph and there is no atmosphere.

Much of the ground within a few meters of the impact point would be vaporized, and a cloud of molten rock would billow out of a growing crater. "At first the cloud would be opaque and very hot, between 50,000 K and 100,000 K," explains Melosh. "But the temperature would drop rapidly. Milliseconds after the initial blast, the cloud would expand to a few meters in diameter and cool to 13,000 K. That's the critical moment," he says, "when the vapor becomes optically thin (transparent); then, all the photons rush out and we can see a flash of light from Earth."

SteveS