My incubator is set at 89.2 degrees. I like incubating the Ball eggs anywhere from 88 to 90, which is why I set mine in the middle. This leaves room for error, but my Helix keeps the temp pretty darn consistant. If you are going to error, error on the low side.
I use a walk in closet as my incubator, with a space heater attached to my Helix temperature controller. I have rubbermaid shelves, with styrofoam boxes. I use periolite and vermiculite as medium, and definitly error on the dry side. I have glass covers to put over the boxes.
Dave