It?s calculated in three steps:
1. You calculate the total amount of your and opponents relevant skills and the skills you count are these:
- strength vs endurance (opponent) ? for damage
- dexterity vs defense (opponent) ? for hit chance
- charisma vs charisma (opponent) ? for chance
You determine total amount of those two skills. let?s say 200 strength + 100 opponent?s endurance = 300 total
2. You take your strength 200 and divide it with total 300 and you get 200/300 = 0,6666. That is the percentage that you will use of your total bonus damage that is shown on your overview page. Also if opponent has some equipment that takes away bonus damage you have to calculate it in your total amount of bonus damage.
3. Use the percentage that you will use 0,6666 multiply with your Bonus damage let?s say it?s 280 and you will get 0,6666 * 280 = 186,65 and that will be the amount of bonus damage that will be shown in your detailed fight report and on that is added the basic damage and determins the total of your damage.
Same calculation goes for hit chance and for chance.
The formulas are mine personal conclusion and there is a possibility of not fully matching with game, specially in case of updates.