Fertile eggs - means rooster chicks..50%, day old pullets means much closer to 100% pullets.
I am going up to morrow for chicks for my broody hen. I set eggs, but none made it. I thought he was not a great rooster.
I hate breaking a broody. I either give her chicks or eggs. I do not expect 100% success, and yet, I too have never had a bad broody hen. I would give the new one eggs, and if the proven bird later goes broody - I would give her eggs too! haha, I mean how do you resist?
I am not a big believer...