Need help to verify on calculating partial pressure in each gas

I have a given atmospheric pressure of 1700 mmHg and atmosphere compsition of 12% O2, 65% N2, 20% Ar, and 3% NH3. It's been awhile since I took chemistry, but for this physio question, I think I need to use this formula (Px = Ptotal [nx/ntotal]) to find the partial pressure, correct?

But then I need to find moles, so I assume the mass. For example, for how I tried to calculate the partial pressure of O2 was assuming 12% as 12g of O2 then divide to molar mass and get 0.37502 mol O2. And then I repeated the same step to all other gases. Then to find the total moles, am I suppose to add all the calculate gas moles or just divide 100g (which is total mass of all gases in atmosphere composition as 100%) to the added molar mass of all the gases? For what I did was add all the calculate gas moles and got the ntotal of 3.37209 mol (which I get a different answer if I did the other way).

So, going from the original formula, PO2 should be 189.06 mmHg.
Px = Ptotal (nx/ntotal)
PO2 = 1700 mmHg (0.37592 mol O2/3.37209 total moles) = 189.06 mmHg

I am not certain if I got the calculations or the idea right, but if someone could confirm it, then I should have no problem in finding the other gases.