You should have at least 95% in the eggbox at all times from what i've been told, but when you have the eggs buried in the substrate you have to be careful with the eggs getting wet.
If the walls of the eggbox have condensation you're fine.
Get a light diffuser to put the eggs on, then you can soak the substrate all you want and should have almost 100%. Just my two cents i've picked up from many hours of research