put it the highest RPM that produces a noise that dosent bother u. Unless ur pc is overheating, then u should just deal with the noise and protect your pc. I have my fan 50% and i hardlt hear it and my pc dosent get too hot
How are you controlling this though? Through software, or hardware? I'd really reccomend getting a hardware fan controller, one that fits in a CD bay or Floppy drive bay.