Cockerels are a crap shoot, sometimes it will work, sometimes it won't. Sometimes you will raise up a couple of roosters and they all turn out rather grand, and sometimes NONE of them will work.
There really is no set of rules such as: "if you do this, then you will get that" when it comes to cockerels.
Personally I would not add cockerels unless I could process cockerels. It really is not a ratio or number deal - it is the luck of the draw.
I will mention, that three summers I raised up a pair of cockerels in a multi-generational flock of 14 head. I really had no need for two roosters, but it seemed to be working well, and I did nothing about it.
But about late summer, I had a friend who needed a rooster and I gladly gave her one of mine. Thing is, there was a tension in the flock that I was not even aware of until it was gone. It was quite noticeable. So be aware.
Do also know that one never really gets a perfect flock, one must add and subtract birds from the flock as it goes. This goes for roosters too. Just because he is eye candy, or has a special color egg gene...well that will not make up for him being rotten to people or hens.
Mrs K