Yes, the ultimate secrete, the unseen matrix, the Holy Grail - Unit Damage Calculation!!!  OK, well not exactly but close - not going too deep into math, the basic formula is adding all bonuses together for your attacker, adding all bonuses together for defender and then dividing the totals.  Remember all bonuses not only include your attack bonus or defense bonus (GBs + event buildings) but also bonuses entitled to each unit as they face another unit (some Heavy receive attack bonus vs some Ranged, etc), and also bonuses added on the battlefield such as gains/losses to location (plains, swamp, etc), distance from another unit, heat damage, and so forth - one must look at the special abilities of each unit to understand what bonuses or losses are accrued vs a specific opponent.  Once the calculation is done the resulting quotient is located on the Calculating Damage spread sheet shown below (this spreadsheet was created by FoEHints     through thousands of hours of fighting and plugging in numbers, an incredible piece of work - I cannot give enough gratitude to FoEHints - the best video content on the internet about FoE).  The location on the Calculating Damage chart gives the high and low hit points of the range expected, excluding the random AO bonus.  

 

Thus an example is;  An Anti Aircraft vehicle vs a IFV.  Lets say I have 283% attack bonus via GBs and Event Buildings, another 25 attack strength is added resulting from the plains tile location the Anti-Aircraft vehicle is placed on. On top, another 80 attack strength is added for attacking a fast unit, the IFV. This results in 345 + 25 + 80 = 450 attack strength.  The IFV then has 94 + 25 = 119 defense strength The quotient then is: 450 / 119 = 3.78  A look on our diagram reveals: We can expect to cause a minimum of 8 and a maximum of 9 damage points against that IFV. The exact value is then picked randomly between both borders.  The calculation in yellow is how much bonus needed for next level.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

To expand upon this data and applying it to Guild Battle Grounds, FoEHints took his calculations and applied them to Guild Battle Grounds, taking into account the ever increasing level of Attrition occurred with each battle.  The application using the formula;

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                                              A*(1+B)                 A*(1+B+X)

                                           ---------------     =    --------------------

                                              V*(1+Z)                V*(1+Z+Q)

​

                      where as A is players base attack value without bonus

                                      B is players current attack bonus

                                      V is computers base defense value without bonus

                                      Z is computers current defense bonus

                                      Q is increase in computers bonus next Attrition level

                                      X is player's addition attack bonus required for +1 Attrition

​

 Produced into a chart, this gives a close approximation as to what Attrition level you will be able to go up to via Auto-Battle - in other words, the point that using Auto-Battle results in total loss of your Army, represented on the chart by the "X".    This chart results in advising you as to what level of Attack Bonus is needed to get to this described loss point.  Given these formulas, the players defense bonus is not really taken into account here as FoEHints feels it is not the primary driver of winning fights due to the use of Rogues and other factors, such as first start - I feel this is true, however I would not attack defense is useless, as in practice there is a define reduction in troop loss overall.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

So the above chart shows 7 lines representing the attack bonus of your army, the bottom being 30% and the top being 1000% (bonus of line is indicated on right).  The "X" is the point described above, the point of total loss when Auto-Battling and this "X" read on the bottom of the chart, which is Attrition level.  Thus, the "X" on the top line (1000% attack bonus) is 74

                                                             (629% attack bonus) is 56

                                                             (400% attack bonus) is 44

                                                             (250% attack bonus) is 31

                                                             (150% attack bonus) is 18

                                                               (90% attack bonus) is 7

                                                               (30% attack bonus) is 0

The % indicated on the left of the chart indicated how much bonus % you would need to increase your "X" level by one more.

 

For review of FoEHints excellent videos on these subjects are here and here.