Let me go and get my slide rule back out. Like a dummy I put both of them back up last night.
Being a weird human being, I still have my high school physics book and
Torque is defined in chapter 2 on page 55.
Torque is = length of the arm multiplied times force applied to the end of the arm.
In this case the "arm" is the length of your socket wrench or allen wrench measured from where you applied the force to where the screw is.
Couple of examples for illustration
20 inch lbs is equal to a 20 pound weight hung on the end of
a one inch long allen wrench or it is equal to 10 pound weight hung on the end of a 2 inch long allen wrench or it is equal to 5 pounds hung on the end of a 4 inch long wrench
New example of a very critical nature
20 foot pounds is equal to 240 inch pounds because we are now hanging 20 pounds off of a one foot (12 inch) arm. Be sure to get this right
Alum heads on a small block Ford are tightened to about 60 Ft lbs as I remember and this means pulling on a foot and a half long socket wrench with torque scale with both hands really hard. I don't think we are dealing with foot pounds here on a scope.
Now then, one other minor point of not so much consequence, as you tighten the screw down with one fluid motion the indicator will go to 20 and if you stop at that point it will probably take 30 to get it started again (just like starting lug nuts on a wheel. If you tighten to 15 and stop and then check with the screw at static it will register over 20. If you have locktite on the screw it will act as a lubricant and 20 will be tighter than 20 with no lubricant. If you are mounting a scope on a 22 rifle then you may not need 20. If you are working on a 460 magnum whopper stopper 20 may not be enough to keep the scope from moving.
Actually for what you are doing all of this is pretty much not really critical unless this is some really expensive optics. Whether you get it to 18 or 22 doesn't matter - torque wrenches are notoriously inaccurate as I understand them.
If we assume that it is 20 inch pounds not foot pounds then measure your allen wrench or what ever and then divide out to get the weight you need to apply to it. If you got a fish scale, trigger scale bow scale or something you can practice a little with it and the allen wrench until you "feel" what is needed.
If you just like new tools, go and get yourself a torque wrench
or if this scope cost more than the national debt then go and get one.
just my $.02