I recommend setting it as high as possible. This is because the fan
will put your attic into a negative pressure. If you have air
conditioning, any openings between house and attic like for ceiling
lights, ceiling fans, around tub showers, etc, fan will suck ac out of
the house. Any time you suck air out of the house, air will suck back
in somewhere else. This makes your attic cooler but actually increases
your cooling bill for the house. If you are doing work in the attic,
turn the fan thermostat down, otherwise turn it as high as possible.
Being an AC contractor, and having test equipment to measure this, you
couldn't GIVE me an attic exhause han. It acn also cause gas furnace
and water heater flues to back draft! Carbon Monoxide! Yuck! Depends
on your house, how tight is construction, how many openings between
house and attic, how many soffit vents, how big is fan etc.
One idea is to install 2 fans, one blowing air in and one blowing air
out. Attic pressure stays more neutral, less side effects. Then you
can turn the thermostats down without hurting things much.