66cayne,
Take the guesswork out of your change and use this spreadsheet. It explains how to make 8 simple measurements, then enter the numbers into the spreadsheet. The spreadsheet calculates the instant center and several other setup numbers. We used it to get the rear suspension set up properly on our 81 Malibu. It helps you get it right the first time without guessing. Good luck.
http://www.patooyee.com/calculators/4BarLinkV3.0c.xls