Divide the CFM by 300 to come up with the Square Feet of NFVA required and then multiply the Square Feet x 144 Square Inches to get the necessary NFVA in square inches that you need for Intake Ventilation for the Exhaust Fan.
It is on the www.AirVent.com
website, under the brochure about Hip Roofs and how to calculate venting for difficult to ventilate structures if I remember correctly.
Here is the link to the brochure:
Here is the pertinent information for the calculation for you.
How Much Intake is
Needed with a Power
Use the following formula to determine the amount of
intake venting required with a power attic ventilator:
CFM rating of the power attic ventilator 300 = square
feet of intake ventilation needed
For example, 700 (CFM) 300 = 2.3 square feet of
intake ventilation required. (Note: Dividing by 300 is a
predetermined calculation established by the
Home Ventilating Institute.)
To turn that number into square inches, which is the
common method of specifying net free area, multiply
it by 144 (there are 144 square inches in a square foot).
For example: 2.3 (square feet) x 144 = 331 square inchesof net free intake area.