there is no real calculation for that. The damage bonus is determined by a couple things. If it's one hand weap then it's determined by the weilders level. If it's two hand weap then it's determined by the delay of the weapon and the level.
For more detailed information you can find the table at
lucy.fnord.net.
Perhaps the best way to build that would be to check your weapon type, grab your level and/or delay and pull the information from a text file. Alternately you could manually enter the information and let it fetch it for ya too. But then, if I was a genious I wouldn't spend so much time looking for answers :)
***edit***
wouldn't spend so much time editing posts to fix a simple url either lol