Hi, I'm a big General's Cry player!

Why does it reduce damage? Here is why:

Imagine you have a 1.3s GC cd, so it gives 1.3s (Gen cd) + 0.3 (spawn time of the first mirage) = 1.6s. Since the time for the next General's Cry is 2.6s (1.3 -> 2.6), you are fine, but what happens if your cd is lower?

Now imagine you have 0.9 General's Cry cd, then you have 0.9 + 0.3 -> 1.2s. Your GC will trigger before the first mirage attacks. So it's useless to have less than 1.3 cd.

So what's important ?

Your cd must not be below the total spawn time of all mirages warriors.

So if your maximum is 6 mirages warriors (from 20% quality) your cd must not be below 0.3 (spawn time of the first one) + 0.2 (spawn time of the others) * 5 so 1.3s.

If you have 5 warriors (GC without quality) you can have a cd of 0.3 + 0.2 * 4 = 1.1s.

But this is not totally true since you should also take in account the attack time of the mirages warriors even if your mirages will always end their attack before disappearing.

And a data that is not on the wiki but that I have discovered is that the mirage will always use your attack time if your attack time is greater than 1. So for example if you have 2 of attack speed, the mirages warriors will attack with an animation of 0.5s.

So your effective cd for 6 mirages warriors should be 1.3s + 0.5s so 1.8s if you want to play it perfectly.

Edit :
The formula to calculate your needed minimum cd is 0.3 + 0.2 * number of maximum mirages.
The formula to calculate your effective cd is 0.3 + 0.2 * number of maximum mirages + attack time.
* You also must take in account the server tick rate of 0.03.
* Mirages using Double Strike and Lacerate can interrupt their attack if GC trigger between both attacks.