Here is a typical problem:
You have an adapter 42" long and you want to acheive a torque at the nut of 256 lbs.ft.
Your torqye wrench is 18" long.
So, using the above letters:
A = length of torque wrench = 18
B = length of adapter = 42
C = torque wrench setting = ?
D = desired torque at end of extension = 256
So, with your 42" extension on your 18" torque wrench, to acheive 256 lb.ft at the nut you need to set your torque wrench to 76.8 lbs.