Calculating adjusted torque values is pretty easy.
La = Adjusted length of torque wrench, center of handle to center of socket
Lo = Original length of torque wrench, center of handle to center of socket
Adjusted torque value = Lo/La
So, if your original length was 12", and your new length is 15", the adjusted value would be 12/15 = 80% of the specified torque reading.
If your original length was 15" and your adjusted length was 12", the adjusted value would be 15/12 = 125% of the specified torque reading.
Hope this helps.