I'm not gonna steal Jim's thunder but I feel as though I ought to say something.
My entire development was built in the early '70s. All the homes are the same except for a few subtle differences. One item that IS the same for all 17 homes is they have 2 gable-end vents and NO soffit or ridge vents. (I, personally, have soffit and ridge vents but no one else does.) Hence, every one has either a gable vent with a fan in ONE end (not both), or a 'mushroom' roof vent. The bottom line is they have the same 'problem' as you.
Sooooo, I suggest you install 1 fan in the gable on the leaward-side of the house. If you install more than 1 fan, they'll kinda 'fight' each other. You know, one wants to push and the other wants to pull. If and when you get a new roof, have a ridge vent installed, end-to-end. If and when you get new siding, or a few extra $$$, look into soffit vents under the eaves. As far as setting the thermostat, I say 85 degrees. I'm sure Jim will have something to add, Jim???
My best to ya and hope this helps.
PS: I recommend the louver-type vents in the gable. If there's a fan pushing air up against a screen, it's like spitting in the wind. Some air will get through but a lot of it will simply be diverted back into the attic space. With louvers, it's like opening a window when the fan comes on! And make sure the louvers are completely closed when the fan isn't on. IF necessary, 'tilt' the bottom of the louver's frame outwards so gravity helps keep the louvers closed when the fan is off. Grainger is a good place to start for a fan and louvers ...