I just installed a powered attic roof fan. Do I need to block the ridge vents?

[aaronphillips81]

I have a 2100 sq. ft. attic and just installed a powered roof vent right in the center of the attic. My question is do I now need to block the ridge vents to keep the fan from pulling air in through the ridge vents instead of the soffit vents? Also, whats the best temp to keep it on? I have it set for 115 right now. I'm not sure if the extra electrical cost to run the fan would outweigh setting the thermostat any lower...