Since you raised silkies years ago, and raise them now, did you notice a very drastic change? The crests are much bigger than the standard called for. Seems they are getting less and less chicken like.Yes. I count my hatch rate based on fertile eggs too.
I honestly believe one size does not fit all when it comes to good fertility. So many factors play a role and basically it will also come down to where in the world you live "this time of year". I collected first laid pullet eggs for this batch in the middle of January. This is dead of winter here. ALL were fertile.
The quitter I tossed last night was a small egg. But it candled before it went into the incubator with a yolk and good air cell. It quit at day four. The eye of the embryo and vein mass clearly visiable in the eggtopsie. Two questionable eggs that may get tossed tonight are normal sized eggs and candled the same. There are ten pullet eggs showing normal development.
Fertility can be lowered for many reasons. Too much fluff on the bum of a hen or a roo is breed specific. Too fat of either the roo or hen. Good eyesight which is compromised on heavily crested birds. And lack of cordination, experience, or willingness from either sex can play a role.
I like my boys to see and my girls to at least see enough to get around.