DOF, in a real camera, is typically determined by the combination or the type of lens used, it's width, and the F-stop.
In the digital world it's a little trickier because you can cheat and achieve DOF's not possible in real life without a lot of very complicated lenses and impossible F-stops. in a real camera altering F-stop also alters the exposure ... whereas in octane F-stop only changes the DOF.
Basically it's up to you. If you're wanting something more realistic I'd suggest watching some youtube videos by Hugh Fenton, or Roger Deakins as they explain cinematography.
I recommend watching this vid:
https://www.youtube.com/watch?v=KwtpJ3T8eK4 .. as it explains a LOT about achieving certain looks.
Gaining knowledge on what's possible with a real camera helps maintaining realism. But when it comes right down to it .. there's no right or wrong way. There are ways in which you'll make the image more surreal .. or more uncomfortable .. or the opposite depending on the lighting you use and the DOF. So really it's down to what kind of feel you want .. and what you're trying to say.
Just my two cents worth.
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