Stacking penalty with > 2 sub flotillas

[Wellington]

I have on several occasions noticed that he tooltip says stacking penalty during battle when my German subs has encountered UK fleets in the Atlantic.
I have no idea what is the most ideal stack of subs but with more than 2 in a stack, one get a stacking penalty?!?. I do not use any admiral that can control less than 6 sub flotillas. Why do I then get the stacking penalty?

 

[Alex_brunius]

I have on several occasions noticed that he tooltip says stacking penalty during battle when my German subs has encountered UK fleets in the Atlantic.
I have no idea what is the most ideal stack of subs but with more than 2 in a stack, one get a stacking penalty?!?. I do not use any admiral that can control less than 6 sub flotillas. Why do I then get the stacking penalty?

You always get stacking penalty. If your using more then the admiral can control you will get something else called over max command penalty instead IIRC.