When turning the adjuster stud, both ends of the UCA move equally.  To decrease the measurements, rotate the adjuster stud clockwise into the aluminum (front) end of the UCA.  To increase the measurements, rotate the adjuster counter-clockwise out of the aluminum (front) end of the UCA.

To adjust the RH UCA, I ran the car up on a pair of ramps, then raised the back of the car at the center of the axle until the tires were just off the ramps, stuck a pair of stands under the frame, removed the ramps, then lowered the axle a bit.  I removed the RH wheel for ease of access.  I spread both locking tabs and backed off the jamb nuts. Turning the adjuster stud was not a problem.  When I got to the 4-1/8" measurement, I checked the longer measurement and it was right on.  Tightened the jamb nuts and folded the locking tabs back in place.  Replaced the wheel and it appears to be in the center of the wheelwell.  Lowered the car back on the ground.  Done!

NOTE:  I don't know where I found this picture, but it is not the axle on my 1995 Impala SS.  The right rear spring had been removed for ease of viewing.