Melee-damage is calculated as the following:
damage = (sword-damage + strength - armor - 1)/10
If you land a critical hit, it is not divided by 10 and each hit will do at least the minimum damage as defined in the scripts (Constants.d or AI_Constants.d), 5 by default.
This is not defined in the scripts. To change it, you'll have to use LeGo (or at least Ikarus+HookEngine.d). In the next release of LeGo, an easy way to change it will be implemented. If you desperately want to change it, I can give you instructions on how to do so.