"Just look at the opening and if it's close that's good enough. " That's the answer I was given when I first asked the question. " good enough " That bothered me, I knew the minimum allowed by law was 10 % and any more could  potentially be a waist of energy so I was determined to find an easy way to verify the setting. To set up an economizer to a given percentage all you need is a thermometer. With a thermometer in hand take the outdoor ( OA) and return air ( RA ) reading and multiply each by the percentage required. If we wanted 10% fresh air we would multiply 10% x OA =T1 and 90% x RA =T2 then we just add T1 + T2 =MA and get our mixed air temperature. If we had a 40oF outdoor air reading and a 75oF return air reading we should set up the minimum position on our economizer to a mixed air temperature of  71.5oF.    ie.   0.10 x 40oF = 4oF,   0.90 x 75oF = 67.5oF,  4oF + 67.5oF = 71.5oF