There is no set torque specification.
Install the castle nut and hand tighten it down. Then use an appropriate castle nut wrench to further tighten it by hand. You should be using either a padded vice or if you don't have a vice by laying the lower receiver on it's LEFT side with the buffer tube off of the end of the table. Have a buddy hold the receiver down so that the pistol grip acts as a stop while you rotate the castle nut with the wrench to further tighten. This will not damage your receiver.
You only need to use the leverage of your hands and the wrench itself.
Once that is done the proper way to fix your castle nut is to properly stake it
not use lock tite or thread locker.
If you look at the barrel end of your castle nut you will see small divots cut into the end of it. Those are the staking points where you take your punch and stake it.
The only time you should use thread locker on it is if you have something like an aftermarket single point sling or adapter receiver end plate that is made out of harder steel than the standard one and it is too hard to stake with the castle nut.
What I posted above is what I do, and is taken straight out of one my AR15 Gunsmithing guide books which I happen to have here on my computer desk. Gunsmithing the AR15
by Patrick Sweeney
TACAV: Griz, if you had to choose between bacon or coffee...
grizcty: You left out the choice of death