Day 16 if candled, a good egg will have a dark pointed tip. Do not lockdown on day 18 if the pointed tip is clear. This means too much fluid in egg.
Below is an egg on day 13, so you can see what a clear tip looks like...
Below is how an egg should look on day 18 and no clear should be seen...
I have many tips on foam incubators if you look in the forum I put it up a few days ago. But if you use a foam incubator, remember to put it on a flat surface, (Not a rug) there are small air holes at the bottom that need to breathe, and draw air.