I can read this two ways:What proportion of roosters should I expect will need to be culled?
"What proportion of [the current cockerels] should I expect will need to be culled [to work with my current number of hens]?" That would be a question about the current situation.
"What proportion of [a batch of cockerels that I buy in future] should I expect will need to be culled [because they are unsuitable for one reason or another]?" That would be a question about planning for the future, to know how many male chicks to buy if you want a good chance of finding one that works.
I don't have anything helpful to suggest either way, just got stuck wondering which question you were actually trying to ask. (I initially read it the second way, but then several people answered it the first way, and now I don't know which one it was supposed to be.)