Wanted to chime in and confuse you all even more
There are of course also hidden calculations and mechanics going on in combat. One of note is called "overwhelm factor", which simulates encircling enemies, but which to some point also counteracts SBDE. The overwhelm factor basically compares the total attack damage of the attacker with the total defense damage of the defender, and the higher the damage of the attacker compared to the defender, the closer his damage output comes the theoretical maximum of 100% (plus x-factor). The maximum is reached when the attacker's damage is 20 times higher than defender's damage. If the stack's damage does not exceed the damage value of the defender or is even smaller, it can get as low as 30% of the theoretical maximum (+x factor). That is also the reason why many players observe in normal fights that their units deal less damage per tick than what is shown in the unit details, because the values shown are the theoretical 100%, before x-factor.
If we put both overwhelm factor and SBDE together, the most efficient army (regarding damage output) would be a large stack consisting of many different unit types, to reach a high stack damage while not lowering SBDE. But one can have many different goals in combat. If the goal is to get over with combat as fast as possible, forming multiple stacks is the way to go, as attacks go out faster from both sides.
Oh and even all what I just wrote is only a simplification of how combat is actually calculated, and there are other factors as well. So please still don't take it literally
There are of course also hidden calculations and mechanics going on in combat. One of note is called "overwhelm factor", which simulates encircling enemies, but which to some point also counteracts SBDE. The overwhelm factor basically compares the total attack damage of the attacker with the total defense damage of the defender, and the higher the damage of the attacker compared to the defender, the closer his damage output comes the theoretical maximum of 100% (plus x-factor). The maximum is reached when the attacker's damage is 20 times higher than defender's damage. If the stack's damage does not exceed the damage value of the defender or is even smaller, it can get as low as 30% of the theoretical maximum (+x factor). That is also the reason why many players observe in normal fights that their units deal less damage per tick than what is shown in the unit details, because the values shown are the theoretical 100%, before x-factor.
If we put both overwhelm factor and SBDE together, the most efficient army (regarding damage output) would be a large stack consisting of many different unit types, to reach a high stack damage while not lowering SBDE. But one can have many different goals in combat. If the goal is to get over with combat as fast as possible, forming multiple stacks is the way to go, as attacks go out faster from both sides.
Oh and even all what I just wrote is only a simplification of how combat is actually calculated, and there are other factors as well. So please still don't take it literally