I've also jacked each side and scribed a line somewhere near the middle of the tread and measured front and back of the tire, again at the center line of the spindle.

Any of these tape methods 'assumes' that you have straight line access between the tires and missing any engine or chassis obstructions. If there are obstructions, then you have to make up another tool, usually a rigid tram bar with extensions/pointers, again to the CL of the spindle - like the old time alignment shops did before computers and lasers.