I have TME sport springs on the car and figured it would be a good idea to install an adjustable track rod. I bought the IPD rod. IPD provided no info on how to measure the rear axle offset. I visited their website to see if they had instructions that I could download but I found nothing.
It first seemed to me that the car should be level and I should hang a plumb line right at the center of the wheel. My garage floor slopes to one side a little bit. Now it occurs to me that it shouldn't really make a big difference. I should get about the same total either way, shouldn't I?
I was also first thinking that the axle should be off the ground with no weight on it but that doesn't really simulate real conditions. You don't drive the car with no weight on the axle. So it sounds like the car should be on the ground for the measurements.

Can I just leave the car on the ground, not level, measure from the plumb line to the center of the wheel on both sides and adjust for half of the difference and be reasonably accurate? What's the proper way to do this?
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'80 DL 2 dr