My technique is to square the rear end to the front end and not worry about squaring the rear axle by itself. I jack up the car at both ends and level it. Put the rear jackstands under the rear axle so the springs are loaded Drop a plumb line from the lower inner control arm mounting bolts and mark the floor. Then drop a line from the rear axle at a consistent reference spot like the rear lower coil-over mounting bolts and mark that point on the floor. Then measure longitudinally and diagonally between the front and rear marks. Adjust your rear 4-bar until the rear is square and perpindicular to the front suspension and the wheelbase is where you want it. Then set the pinion angle with the upper rear trailing arms. At the same time, you will be adjusting the panhard bar to help square everything up as well.