[Charles Stewart]

"Typically, we ASSUME a constant percentage of oxygen (20.90%) within that density. The only way to determine the actual percentage of oxygen and therefore the actual amount of oxygen per volume is by specifically measuring the oxygen level independently from the temp, hum and press."

Thanks Mr. Bartoli for your precious input.
That was exactly the purpose of my question. I doubted something was missing in my weather calculation and I was also assuming a constant percentage of oxygen.
keep on coaching us...