Don, as long as the torque wrench is used at a right angle to the adapter then calibration isn't needed.
Clymer manual has this formula for all other cases:
TW = (TA x L) / (L + A) = E
TW = torque setting on wrench
TA = actual torque to be applied to nut (ft-lbs)
A = amount that adapter increases or decreases effective lever length. Measured along centerline of wrench from center of the drive to center of adapter box end (inches)
L = length of lever or adapter measured along centerline from enter of the drive to center of grip (inches)
For example, let's say we want to apply 20 ft/lbs of torque to a nut using a wrench that has an 'L' of 14 inches and an 'A' of 3 inches. TW = (20 * 14) / (14 + 3) = 280 / 17 = 16.5 ft/lbs. So, reset your torque wrench from 20 to 16.5 ft/lbs when using this wrench and adapter.