To be technical about it, and assuming you are on stands or a hoist:
1. Tighten the nut quite snuggly,
2. Use a ratchet and a pipe extension on the handle.
For 20 foot-pounds (or pound-feet, if you insist) hang a 10 pound
weight on the pipe two feet from the socket, with the bar sort of
level*.
If the nut turns, repeat the step 2
3. When the weight stops dropping due to tightness of the nut, you have
your torque.
* You can use a 20# weight on the bar one foot from the socket, of
course.
This all ignores stiction, so give it a good wack!
Karl
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