We downloaded this MicroSoft Excel file which was a big help in understanding, and getting the rear edge geometry set up in our 81 Malibu. All instructions are in the file. Measure the end point locations of all rear control arms and enter them in the spreadsheet. It will then calculate and graphically depict the Instant Center location and several other critical dimensions. You can then alter the numbers to see what you need to change to arrive at the Instant Center you desire. Good luck.
www.patooyee.com/calculators/
4BarLink
V3.0c.xls