Chassis Guys, a little help please?

How do you determine exact center of the car?

I plan to get back out there and check the centering of the rear axle, but this time using the inside of each frame rail, not that I expect it to make a difference. I'll then check the shocks for straightness as they are used shocks and could possibly be bent, but I don't expect to find anything.

This totally took the wind out of my sails, what little bit I had. I was so proud when all of my previous measurements came out perfect.