90% relative humidity is a good number to strive for. The important thing is to keep them hydrated and warm. High humidity helps a bunch, but you need the heat, minimum of 80*F, and access to water at all times. Soaking them once or twice a day is a good idea too. In the wild, babies are hatched during a period when the relative humidity is from 75% to 100%. Since they are low to the ground and hide amongst the thick foilage I bet they stay pretty wet most of the time.
Basking temp should be between 100-105, warm 95, cool, 80-85, your humidity should be at 80%. If you click the threads below my signature it will give you the correct temps and humidity...
I recommend a closed chamber. It is very difficult to maintain good conditions in an open topped box. Here's a thread for you. Been using these for a long time and the results are fantastic. http://www.tortoiseforum.org/thread-32333.html
It will be tough to get high humidity in an open top enclosure so like Tom suggested, i'd strongly recommend covering it. I also recommend getting a cheap humidifier from Target, Walmart or anywhere that carries them because without it i've found that it's difficult to get the humidity up there where it should be. As far as a goal for how hight the humidity should be is concerned there really isn't an upper limit, 100% will have no ill affects, however, you must make sure to be festidious about maintiaining the necessary high temps so as to prevent respiratory infections.