My daughter and her husband live an hour's drive from Edmonton, Alberta.  Winter temperatures are frequently -30 and even -40 degrees F.  When we were making some changes to the duct work in the smallish one story house, built in 1955, (they converted 3 small bedrooms into 2 larger ones), we noticed that there was a 5" duct bringing outside air into the return plenum.  Apparently this is required by modern codes,  (not sure when this heating system was put in, but probably not  when the home was built).  I can see that when the dryer and/or the bathroom exhaust fan were working that an outside air supply would be a good idea.  However, bringing in too much outside air would cause the heating bill to be higher than necessary, especially at - 35F. 
I presume that the damper in the 5" fresh air line needs to be adjusted to prevent excessive air from coming in.  The question is, how do you know where to set the damper?  Would taping a plastic sheet over an open window and watching for the plastic to be in a neutral position (i.e. neither bellowing in nor out) with the clothes dryer on, be a guide when adjusting the damper?
My daughter lives in a small town far from any city where expert help might be available.
Is there an easy way to know where to set the damper without any fancy equipment, or do we have to bring someone in from Edmonton?