I think this is the average way of determining hatch rate;
1. Early deaths are removed from total set.
2. Infertile are removed from total set.
3. What remains is what is locked down (so if you cull on day 18 candle and decide its clear, its an infertile, and doesn't count)
4. What hatches, is hatch rate from what went into lockdown.
Some ppl compare all they got to what they go, and that's not a hatch rate.