Normal P/F ratio=600.
As a generalization :
everybody breathing room air (21%) will have an PaO2=100. P/F ratio=100/0.2 = 500.
everybody breathing 100% oxygen via NRB will have an PaO2=500. P/F ratio=500/1=500.
so now, you can figure out the max PaO2 on any FiO2.
FiO2 P/F ratio PaO2
100% 600 600
90% 540 600
80% 480 600
70% 420 600
60% 360 600
50% 300 600
40% 240 600
30% 180 600
21% 120 600