Evaluating hypoxia - not that tough!

I had a few queries from you guys about the calculation of partial pressure of Oxygen in alveoli (PA02) , in my previous post.
Thanks for those who brought this up
This is what I mentioned for calculation of A-a gradient
A-a = (713 x FiO2) - PaO2-PaCO2-PaCO2/4

where PAO2 = (760-47) x FiO2 - PaCO2 /0.8
          here 760 is the atmospheric pressure...and 47 is water vapour pressure....and 0.8 is the respiratory quotient assuming we have the stipulated proportion of fat,CHO & protein in the diet.
This is how I got 713.   I have simplified the PaCO2 /0.8 portion of this equation into PaCO2 - PaCO2 /4  just because I find this easier than having a 0.8 in the denominator.

Usually A-a gradiesnt is calculated by PAO2 - PaO2 , .....and PAO2 is taken as 150 . This value of 150 is only applicable if the patient is breathing room air i.e 0.21.  If not then we have to substitute that value and arrive at the PAO2 appropriate for that FiO2. This is the reason I didnt mention the value 150 in the equation.
Hope this clarifies!