Ideally, what should the attic temperature be compared to the outside temperature if there is balanced attic ventilation?

A good rule of thumb is the attic temperature should be within 10-15 degrees of the outside temperature.

A good rule of thumb is the attic temperature should be within 10-15 degrees of the outside temperature. No more than 20 degrees. An optimally designed attic ventilation system should be able to maintain this temperature difference in the summer and winter. Of course, the more extreme the temperatures the less likely the attic is to be within this range.