To calculate the proper amount of attic ventilation needed you have to have the correct amount of Net Free Area (NFA). NFA is the actual ‘free air’ opening of a roof or soffit vent after allowing for the screen or framework of the louver. To achieve the NFA it is necessary to cuto the proper size opening in the roof deck or soffit.
Use this formula if one half of the ventilation is soffit and one half of the ventilation is in the roof or if a vapor retardent having a transmission rating not exceding 1 perm is installed on the warm side of the insulation.
To learn how to do the calculations click this link to go to Calculating Proper Attic Ventilation