Torque wrench tightening yields the broadest range of bolt preload. Because torque is sensitive to variables like cleanliness, lube, washers & technique the only way to truly know is by testing with all the variables controlled. This is why you will typically see specific instructions as to what type of lube to use. Torque wrench accuracy is another variable.
Bolt stretch is directly proportional to the bolt preload until you get to the yield point. So when you measure stretch you are directly measuring the preload. This is the best method if you have the right tools.
The K1 instructions fall between these two as far a accuracy and ease of procedure. The light torque of 20 ft-lbs simply brings all the members into firm contact. The additional turn of 60 degrees provides the usefull preload. Because the manufacturer knows the thread pitch and geometry of the connection, they know how many degrees will result in the correct preload. Because the torque wrench is used to a low value the range of error is minimized... 10% error at 20 ft-lbs is alot better than 10% at 80.
