...I used the number 2%, not 2.5%, in my calculations.

In reality, the number 2% wasn't invented by you. There are several sources that provide this specific reduction coefficient, and you may have adopted it as a guideline at some point... Previously, I also used it in my calculations and attributed any discrepancies to internal rounding errors...

Practice has shown that this coefficient is incorrect, and, as you know, there is no original source that explains the internal mechanism of calculating most game elements. The developers didn't deem it necessary to share this information with players. However, there were interviews published on Progamer with one of the game's programmers, who shared many interesting details (in particular, he clarified the intricacies of diplomacy)... By using this information, along with players' own observations, we can sometimes uncover the truth :)

It was these circumstances that prompted me to write a guide to the physics of the Heroes world. Many players on various forums provided great support by sharing their knowledge and observations.

Regarding the formulas. I deliberately didn't include formulas in my post because I was explaining the calculation mechanism itself, which seems more useful than just a formula. Your observation can be explained by the fact that the "defense bonus" of the target (when defense is higher than the attacker's attack) affects both the attacker's damage and their secondary "Attack" skill. Therefore, the multiplication doesn't violate the calculation mechanism. The "attack bonus" that you mentioned doesn't indirectly affect the attacker's secondary "Attack" skill. ("Attack" only modifies the base damage and therefore should be considered separately)... This is what your observations have shown. However, I admit that this formula may not always accurately reflect the actual damage.

To provide a complete formula, it is also necessary to consider the interaction of the secondary skills Attack and Ranged Attack for ranged units (they exclude each other), the effect of the Bless and Accuracy spells (they complement each other), which can also be used and disrupt our calculations, and so on. In addition, there is the concept of Luck, which only modifies the base Damage but doesn't affect bonuses, and much more.

Try to calculate the damage that a Hydra will inflict when attacking multiple stacks simultaneously using this approach... It won't be so simple :)

This can only be verified experimentally.

Sincerely,
AmberSoler