Its a very difficult calculation and can be missleading! I have seen adverts advertising hundreds of 1st's to find out the birds positions were 1st's to the loft, beaten by nestmate etc and not club or fed 1sts.
A very good friend of mine built his show team up over nearly 20 years, when he started would breed 6 yb's from his main pairs and prob keep 2, the others would be culled, over the years and through selective breeding this went up to having 5 good from 6.