Thanks to the simplicity of the mechanics for siege weapon bonuses, I was able to make a [plain] observation of the spy level values.  Your CA value is based on this:  1 point per spy.  This is then multiplied by 2^spylevel.  So...if you have 3 spies and level 3 spy skill, your CA is 24.  Simple enough...unless you're an elf.  Then you can add 25%.  This may seem ordinary...but if you spy on someone, you can now [easily] figure out their CA, even though you didn't get it in the report.  Sorry if this takes the magic out of watching that number balloon...