A NPT is a tapered thread that seals in the thread area, with the help of tape or dope most times. I have seen some fitting that thread into an female npt thread and seal with an elastomer seal like that acts like a face seal, but I would not rely on it seal anything but air. They can be be reused a couple of times, after that the elastomer is trashed. If the thread isn't a little past the high to low limit they don't work well either.
The Machinery handbook gives a dimension for hand tight I believe. Additional tightening after that is determined by feel most times, and driven by the material of the threaded components. Softer materials can be threaded more turns but you have to careful not the split the female thread or shear off the male end.
Not really an answer to your questions, but there really isn't an answer that be correct all the time.