Every breechplug installation will vary in torque values.
If indexing is required, such as an installation on an octagonal profile barrel, the location of the tang relative to a corresponding flat will dictate the required force to "sock it down".
The most critical factor involves positive contact with the face of the breechplug with the inside shoulder of the barrel. This must coincide with the index, tricky at first, but easier every time after.
Once you are positive that proper contact is achieved, it should require some force to tighten. A 4 foot breaker bar is too much, and a 4 inch Chescent (Chinese Crescent wrench) is too little. You should definitely feel it tighten up, but not so tight that you feel that the barrel might twist. You MUST have a solid bench and vise, as a rickety setup will give a false feeling of the proper torque.
To illustrate, let's say that the tang is one flat short of being aligned AND bottomed out. No way! You would have to call some Cosmonaut buddies over to tighten that one. You should only be off about two thirds from indexing after a very light snug with the wrench.
The one good thing is that because of the substantial diameter of the threaded portion of the breechplug, it will stop dead cold, and not permit you to severely over torque it. (Most breechplugs that is, some ill fitting or cast ones might disfigure or break.)
My breechplug wrench is 14 inches long, and I just roll them into place with some good force, but never to the point where I have to jerk the wrench. In actual foot pounds, the figure might run 20 to 30 pounds for a 5/8 diameter plug. a breechplug has shallow penetration, and established torque values for fasteners should not apply. They are plugs, not fasteners subjected to vibration and shear loads etc.
This is a "feel" issue, and no matter what I tell my Brother in law, He will always torque everything to the moon!
This is just my particular take on the issue.
:m2c: