Bear with me through this please:
As an avid shooter and part time machinist I make and modify things. Long story short, I wonder if someone could mate a trigger pull gauge, hell for that matter a fisherman's weight gauge would work too, to an adapter and make a crude, yet VERY adjustable torque wrench.
The formula for torque is: Tau = ®(F)sin(theta)
Where r = length of arm
F = force applied
sin(theta) = the angle at which it is applied.
For simplicity we will use T = r*F
We can keep things at a right angle and ditch the trig.
So slip an open end wrench into a fixture (read - tube with hole drilled across the thing) and measure
the distance between the center of the wrench and center of the hole. Estimate the hole with a nut in it for closer measure.
We'll assume it 22.8inches in length, or 1.9ft. That's our r.
T = r*F > 20 = 1.9*F > 20/1.9 = F
Therefore by exerting 10.53lbf on the assembly you have reached a 20lb-ft torque spec.
Keynotes:
1) Force gauge must be at right angle to lever to achieve accurate measure
2) Lever must be horizontal and supported to remove any errors due to the acceleration of gravity.
3) Make sure your force gauge can handle the force you are using. If it won't you need a longer lever.