How exactly did they compute that constant? I suspect the figure they give is the result of many heated debates and much flaming on mailing lists and whiteboards.
Well theres so much material in the Expanded Universe including a book called "Death Star" that tells the story of the Rebel Alliance victory from the perspective of people who worked there. So, I would just have to assume that they went over the specs/dimensions/weight of the Death Star at some point.
That page itself has conflicting reports of the size. At the top it says "Size: 160 kilometers in diameter," but lower on down the page, it says "The Death Star was 120 kilometers in diameter."
It's also only valid at a certain radius (probably the surface). So the sensor wouldn't be calibrated correctly after you went inside (it would be off by differing amounts depending how far from the center you were).
The same is true of the constants for the planets, but at least they are (mostly) solid and not designed to be lived inside of.
