Board Thread:Game Discussion/@comment-11128765-20130930132628/@comment-11490363-20130930170110

It is calculated based on the damage formula:

damage from boss = A(1 - 3*sqrt(D)/1000)

Where A is the boss attack, and D is the defending unit's defense. You can measure the damage from the boss, and you know D, so you can solve for A.

Note that the boss attack damage does have a slight random factor either a stock 1K range or 3%, not sure which yet), so the value you get will not be the exact value, but it will vary a bit from the exact boss attack value, but not a huge amount.

I am not sure if that data is from calculations or from hacking the binaries to see the stats.