Yes, you have to bring in more outside than you are exhausting to make the building positive. The outside air is typically brought in by the supply fan from the economizer. The amount of extra air being pulled in is the difference between supply fan CFM, and return fan CFM. As Rob mentioned, this can also be done with some % difference in speed, or based on building static pressure measurement, but I find that using actual flow is far easier to control, and it's not like flow sensors are any more expensive than static pressure. Besides, having the flow sensor on the supply fan and the return fan provides another diagnostic data point as well.