Been driving my old Fords with straight axles for a while now and they seem very comfortable with 1/8" of toe in. That is, I put a tape measure across the back as high as I can get, usually constrained by the wishbone depending on the size tire and drop of the axle. Then, at the front of the tires I use the same reference points that I measured from in the back but 1/8" less on the tape measure. Ain't rocket science, but my cars track OK, steering pressure does not seem excessive, and wear seems fine. Wear is the tough one, because realistically, we don't drive these old cars enough usually to tell. That is, the tires usually show signs of cracking and rot before tread wear.