That largely depends on how the breaking up of the asteroid occurs. The SDN asteroid destruction calculator assumes, for instance, that the fragmentation of the asteroid will be as though an explosive were perfectly placed and dedonated at it's center for maximum effect.
Obviously this is not the case, nor is the fact that 20 km wide Dobido is not very likely to be a sphere, dispite what the
comic books show. Dobido is, suprisingly enough, of roughly equal mass as a sphere of all-granite rock would be; 10e12 metric tons versus 9.76e12 metric tons. So all things considered, and not knowing the dynamics of the moon's breakup under a Star Destroyer's bombardment, at least 1 gigaton for a very upper end estimate is not so unreasonable. To be really fair, though, we may even choose to interpert Han's "
the falling pieces would devastate Sernpidal" part of his statement to mean that the SD's initial bombardment might knock still substantial pieces of the moon down to the planet's surface. That leaves open still the possibility of yeilds up to around 8-10 gigatons.
Another way to look at is if the broad side of an SD is around 1-10 gigatons, that would mean shattering a volume of 4.190e12 cubic meters into chunks of around 10 meters or 523 meters cubed. Dividing that into nearly 42 trillion cubic meters gives us over 80
billion pieces of Dobido for the SD to now have to chase down! All of that expanding out away from the original moon's diameter at fairly high velocity! I think in the best case scenario, assuming 100 guns firing every second and all guns having exactly equal firepower, it would take
2.5 years to hunt down every 10 meter piece and vaporize it!
Of course there's all kinds of factors that could change those numbers. So I think that a few gigatons on the upper extreme end of SD firepower is not too far off.
-Mike