If you put the crows foot on 90degrees to the centerline of the torque wrench you do not need to compensate for the extra length. If you have to do it inline here is the geeky engineer method.
C=LW/LW+LA
Where:
C=correction factor
LW=length of original wrench (from center of square drive to middle of your hand when it is on the wrench)
LA=length of adapter (from center of square drive to center of bolt.
Example:
Using a 12" long torque wrench and 2" long crows foot
Torque you want is 100 in*lbs
12/12+2=.857
.857*100=85.7 in*lbs ---this is what you set your wrench to
So, you can see that if you can get away with the crows foot being 90degrees to the torque wrench it is much easier.
Hope this helps
Paul