I need some help in trying to calculate the percentage of oxygen existing in
a closed room.  This is partly just for my interest, but there is a
practical side to my question also.

I live in a recently built house that is one of these "super-insulated"
structures with little outside air exchange except what is forced via fans.
During the winter months I live primarily in a couple of rooms closed off
from the main house to save on heating bills.

I have equipment that very accurately measures the carbon dioxide level in
real time, that is located in my main living area.  I live at 3000 feet
elevation, and the house is all electric;  no oxygen consuming heating, not
even a wood buring stove.  I don't have any way of directly measuring o2 in
the house.

The measured co2 in my living area ranges from approximately 500 ppm to over
2000 ppm.  The higher concentrations generally occur during the winter
months when I am spending more of my time indoors, and am preserving the
heat by running the forced air intake fans less.  (This will be changing in
the future, as I will be forcing air from a solar heated porch into the
house during some winter daylight hours)

My question is, from knowing the co2 concentration (and humidity percent
also) in a closed space, how can I calculate the oxygen concentration?  I
know that the o2 conentration of normal dry air is around 20.95 percent, but
suspect that the relationship is not perfectly direct between the increase
in co2 and the decrease of 02.

Any help on this math problem would be appreciated.

Thanks
Jim