After day 18 you don't turn them.
		
		
	 
You can keep turning them if you want, the day 18 lockdown the no turn thing is mostly in regards to automatic egg turners that can cause harm and injury to hatching eggs and thus should be turned off and/or removed on day 18 just in case you have early hatchers...   You can hand turn them all the way until the end if you desire... Also lockdown at day 18 is a reminder to raise humidity for an easier hatch...
Lockdown has grown well beyond it's intended purpose and scares too many new people with all the black and white claims...  You most definenly can open the incubator during lockdown if you desire as long as you maintain the humidity levels...  This can be done with a spray bottle full of warm water, open it do your things and then shoot a few mistings in the incubator before closing it and the humidity will jump right back up...
Broody birds do absolutely nothing different on day 18-21 that they don't don on day 1, they can't count and thus don't even know day 1 from day 21 until the chicks actually start to hatch...  And even then the hen does pretty much the same thing with the unhatched eggs under her, she never stops 'turning' and moving them until she tosses the shell out from under her...
To the OP, humidity is based on surface area, you fill the compartments in the incubator until you get a humidity that works for you, and that humidity is not some set number it can vary all over the place by location, incubator and other variables....  Experience and monitoring the air cell growth is what you really need to focus on over a humidty number...
As for the red plugs, they should be removed as soon as the first birds pip out of the shell, they need the fresh air at that point as their lungs are kicking in...  You can do it earlier as long as you maintain humidity and temp...