Rule of thumb is 1 sq ft of ventilation for every 300 sq ft of attic space, this is for both soffit and roof ventilation. Here's a link that'll give more details. A continuous soffit vent is really the best, make sure you install ventilation chutes in the rafter cavities in the attic to prevent insulation from blocking the soffit vents.
http://www.homedepot.com/HDUS/EN_US/...entltn_01.html
I'd also leave your gable vents and add your fan to the roof instead, along with more passive air vents if possible. More is better when it comes to ventillation.
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