Just some various thoughts to think about.
If a tire has a certain amount of camber in it, the toe in/out dimension will change as you measure further from the ground, so a fixed toe in/out number has to be accompanied by a fixed height measurement.
Marking and scribing the tire as it spins is a great idea and the tire/wheel should be raised up as close to the tire/wheel as possible (as opposed to raising both sides at the same time) to minimize the inclusion of play in the bearings, steering, etc.
It is my opinion that this accumulation of tolerances, clearances and wear is exactly why it is important to keep the wheel on the ground at all times after the scribe line has been put in place.
My experience has shown that after making an adjustment it is important to roll the car at least one full tire rotation backward, then roll it forward to EXACTLY the same spot each time before measuring again, then repeat the process untill satisfied.
Because each car does not have the same accumulation of clearances, wear and tolerances, it is logical to conclude that all cars will not perform best with the same measure of toe in.
'Rear -steer' cars will lose toe in as the speed increases and the gyroscopic effect straightens up the wheel and absorbs the total of the tolerances, clearances and wear. If your car has more of these variances, you should expect to start with more initial toe in.
Toe in is required to offset these variables and make the toe in be zero at driving speeds to minimize tire scrub and wear.
My experience has also been to set the steering wheel about 1/16 of rotation off center of the steering box when setting toe in so that when you are traveling straight you are not 'wandering' on the play between the worm and the sector.
This 'wander' might not be noticeable at the speeds that Model A's travel, but it IS there.
Again, just thoughts.