According to So Easy to Preserve, “When using two-piece lids, place the treated lid on the filled jar, center it, and hold it in place with fingers. Then screw down the band until it is fingertip tight. These lids should not be tightened further after processing.
The screw band should be tightened just to fingertip tight. It is important not to use force or to use jar tighteners when applying two-piece lids. During processing, air is forced out of the jar. If the screw band is too tight, air cannot escape. Air must be able to escape from the jar during processing. If air cannot escape, it can buckle lids. Buckled lids are deformed in some way by air in the jar trying to force its way out; they may not seal properly. Hot air needs to escape the jar to create a vacuum when the jar cools.
A practical way to determine if the lid is fingertip tight is to place the band on the jar, turn it just until you feel resistance, then turn the band one-quarter turn more. For beginning canners, it may help to mark the band and lid with a marker at the point of first resistance and at the point that represents an additional quarter turn and to then turn the band to that point.
The Ball folks have just released the Ball®Sure Tight™ Band Tool; it essentially is a torque wrench for canning jars. According to information provided on freshpreserving.com, the tool “secures jar bands with just the right amount of torque. Getting a good seal starts with securing the band to the correct force. This tool takes the guess work out of securing the band to ‘fingertip tight’”.