Do you candle to evaluate the air cell development? Some people prefer the weighing method.
Either way the point is that the air cell needs to enlargen to a specific size by day 18. Because THen humidity needs to be elevated for lockdown and further development of the air cell slows or ceases all together with higher humidity levels.
I candle at day 7, 14, and at 18 for lockdown. I look for veins/embryo development, bloodrings, check for smell, and to make sure that the air cell is getting bigger. I have an excel spread sheet that I print off and write down what I observe. Anything that doesn't look right I separate in the bator. If it looks worse, smells or I find clear ones by day 18 they are removed.