Hi, I would like to know how siege damage is calculated. I mean the damage you do while in orbit to bring defense down to zero, not the damage during a ground invasion.

For instance, titanium slugs seem to do a flat increase of damage, but how is the rest calculated?

It seems to be largely based on the number of CP you have in a fleet. If you count your CP and double it it will give you your seige damage in most cases. There do seem to be some caveats though. You cannot issue a seige/blockade unless at least 1 ship has a space weapon module. I demo this in my latest Horatio livestream video, where my fleet decked out entirely with Titanium slugs is unabvle to issue a blockade/siege until i move a single explorer ship with a single slug cannon into it. 

I have also seen siege damage get recalculated multilpe times, where for some reason it starts out much lower than it should, and increases a bit after a few turns. I have no idea what that is about but have caught it on camera a few times as well.