ok, I finished last night and ran distances behind and in front of both axles. I register no toe on the front and about 1/16" total toe out in the rear. When I use the laser to measure toe on each wheel I get zero up front, expected, and almost zero in the rear, close enough.
I align the laser equidistant with the hub in front and in the rear, and a perfectly centered steering wheel.
I made the assumption that the distance from hub to hub on the front is axle is equal to the distance between hub to hub on the rear axle.
Double checking with a tape ruler behind and in front for total toe confirms I can do that on the C5.
In all these measurements, placing tape measure on uneven ground, reading level bubble perfectly centered, calculating center of vehicle perfectly, etc. there are errors that creep in. If you were to add up all the slop I'm guessing that we could be off by at least a 1/16" if not more in any direction. Hence, I'm not so confident any one method provides a high amount of accuracy. Precision yes, repeatability, but absolute doubtful.
I agree my method does not take center line into account, hence thrust is in question. I don't have a long enough straight edge to check what my thrust departure is, if any.