I keep my humidity between 35-40% of I can. Keep in mind humidity is only a tool to affect weight loss in the eggs. You can gauge this by checking aircells or weighing eggs.
As for lockdown. To answer your question you must understand why people lockdown. It's to allow the chicks time to get into position for hatch. And increase humidity so that when they pip externally they won't dry out between pip-zip. There is no set day. The reason day 18-19 gets thrown out is to make sure the humidity is correct for any early hatchers you might have. I've seen posters "lock down" from days 15-20. If this is your first hatch day 18 is a good starting point.
Thank you! Can I rely on the air cell to tell me also? Like in the photos below