It's going to be hard to figure out the humidity and with the egg being covered you can't tell by what the egg is doing (ie becoming sunken in).
If your getting good temperatures with using the heat mat, than you can use a smaller container (either clear or with an easily removable lid). Fill container half full with glitter grade vermiculite or perlite (mixed weight wise one to one with water). Moving the egg now won't hurt it as long as you don't turn it. Place it in the vermiculite/perlite half buried. Depending on the size of the container, you can put 1 to 4 holes in the lid.
Place everything in a dark spot or as dark as possible. With the high humidity light will cause algae to grow (and possibly to grow on the egg) which isn't good for the egg.
I've hatched 100's of box turtle eggs basicly this way
Danny