sump pump depth

I have a home with a sump pit in the basement. It is 34" deep from the garage floor. How close to the top of the pit should I allow the water to rise before I have the pump kick on? After 3" of rain on already soggy ground this past weekend the water level will rise to about 12" below floor level where it stops. I do not want to keep the level way down if this 12" is sufficient to keep things dry. However, I do not want to damage the house trying to save on the electric bill and pump wear. Any advice is appreciated