Mine always seem to hatch anywhere between days 18-20. I just keep a very good eye on them. I don't do a true "lock down". I minimize how many times I open the bator, but since most of the time I am running stagard hatches, I have to open the bator to turn the other eggs. I just mist the eggs with warm water if I have to open the bator. I have found that sometime the TINY TINE eggs need some "help". I will not step in until we are reaching the 36 hour mark and the chicks are starting to show signs of loosing strength. Then I will carefully remove portions of the shell over a few hours, making the chick do some of the work in between me helping. If I do have to help, I wrap the egg loosely in a warm wet paper towel. I NEVER remove the membrain, just the shell.
Good luck with your hatch ~ we want to see pictures of the new itty bitties!