I guess the easiest way out would be to install a fresh air supply kit directly to the fireplace.
You can have an airflow test performed to see the actual #'s, but if you're in the sticks, you might have a problem finding a company to perform such test.
The standard is to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant.
http://www.airkinglimited.com/pages/exhaust/ashrae3.htmlRight now I am in the same situation, our foam house is so tight, I'm heating 2500sqft with a 80,000btu propane furnace only ducted to the basement... lol. The leakage through the subfloor is sufficient in keeping the main floor and loft comfortable. I will be running a direct air supply vent to our woodstove, and installing low cfm exhaust fans in bathrooms and kitchen.