I have done some calculations based on the algorithm exposed Tridus and assuming that when a number between 0 and N is rolled, all integer values from 0 to N have the same probability. The results are roughly the following (if my calculations are correct):
If you are facing an opponent with 0 DEF, you will roughly do an average damage of your ATK/2 per swing.
If you are facing an opponent with the same DEF than your attack, you will roughly do an average damage of your ATK/3 per swing
if you are facing an opponent with a DEF equals to 2 times your ATK, you will roughly do an average damage of your ATK/6 per swing
if you are facing an opponent with a DEF equals to 4 times your ATK, you will roughly do an average damage of your ATK/12 per swing
