For a proper ratio of males to hens fertility should be nearly 100%. Your 67% fertility rate is low. Not all cocks will service more than one hen.Do you remember when I had 8 hens and only one cock? He had five girls that were his main girls and three that he mostly ignored and chased away. However, he worked hard to fertilize all of the girls eggs, I even saw him mate the chased away ones. I expected low fertility but it was around 67%! Now that I have a less skewed ratio, he keeps two of those original hens in his harem, but the rest of his original hens have taken up with other cocks.