How tight should passengers wear the seat belt?
According to this document published by the FAA (about smaller GA planes but the topic is applicable) 


  Tests have shown that slack in the restraint system should be minimal.
  In an impact, your body keeps moving until the slack is taken out of
  the restraint, but then must be abruptly stopped to “catch up” with
  the airplane. The restraint should be adjusted as tightly as your
  comfort will permit to minimize potential injuries.


Here is a video of them testing the seats. You can see the waist lines of the dummies does not move very much. 

Here is an interesting research paper on the topic, may be worth checking out.

A study was conducted by the DOT "Human Factors Associated With the Certification of Airplane Passenger Seats: Seat Belt Adjustment and Release" on the difference between belt tension (as applied by a passenger) in both a cruise and emergency setting. This looks like the most conclusive numbers I can find on measured tension.


  An analysis of results indicate that most passengers (90%) tighten
  the lap belt to a tension less than 7 lb. during normal flight
  conditions and less than 10 lb. for an anticipated emergency.