Toe needs to be done off the rim not the tire
What you need to do is create mounting points so you are working off the cars center line.
We use two bars one mounted in front other in rear and run string between them this way you can do toe on four wheels.
Very nice job noid.
I have one similar but nowhere near as nice as yours.
Take tires off the vehicle
Clamp 3'' angle iron to each brake rotor.
Measure front to back easily and quickly - and most importantly, accurately.
You can certainly do it off the tire; just make sure not to hit lettering. Traditional alignment machines need to reference off the rim because there is no percise way for the reference plates to clamp and move with the tire.
If paranoid you can measure the tire runout before proceeding.
Re: center line
Its a waste of time trying to find center. Its best to turn wheels lock to lock (turn plates make this easiest) to confirm the same amount of degrees are achieved on both locks. From there both sides should be adjusted in equal amounts for desired toe in/out.
Even easier, many cars have a center indicator at the steering rack knuckle. Although I only trust it if the rack is original.
I was taught over 40 years ago and you will find major disagreements in your method. Some of the people I have worked with and been taught by have published books that are very well accepted in the industry. carry on