DIY Vehicle Convergence Device: Toe-In Adjustment, Rear & Front Axle Wheels Alignment Procedure

I warmly welcome.
During Sunday boredom, I came up with the idea to make my own vehicle convergence device.
But before I start building, explain the exact convergence procedure to me.
In my opinion, to set the correct toe-in you should check the position of the rear axle wheels, if we already have the wheels parallel to each other, then we set the front axle wheels, should this be how it should be done?

Do you want to make a device for something you don't know about? Where's the sense here? The first thing you need to do this is a well-leveled, even floor around the channel. The essence of setting "convergence" is to achieve parallelism of the wheels - the distance between the points at the rear of the rim must be the same as at the front - or deviated by the value specified by the vehicle manufacturer. This value is defined as the difference in mm or how many meters the track of the wheels will go down / go over a distance of 1km. However, what you write about is different - it is the measurements of the floor plate geometry - whether the wheels stand in line, which is not normally determined - only in the event of accident repairs.

And here is a makeshift device design modeled on existing ones.
Blue and red are lasers, and green are appropriate screens with graduations in mm

Added after 3 [minutes]:

Parallel wheels of e.g. the left front wheel and the rear left wheel?

How do you mount the lasers so that they are so accurate that you can set the convergence in tolerance?

This is almost impossible. At home.

The convergence or divergence of the front (and also the rear) wheels in the conditions of a "garage" workshop can be checked quite accurately using a telescopic ruler / pole. I used to do it with a Dacia 1300P using a long enough telescopic antenna and a long ruler / ruler.
The distance between the edge of the rim, e.g. the front wheels at the rear (behind the axle) and the front is measured with a "telescope". The difference in millimeters is quite precise.
In any case, it could not be adjusted correctly, there was no removal or serration of the tires. Of course, this is a very approximate method, but it works, for example, when replacing a ball pin, you only need to measure "before and after", you can definitely correct the difference.
A channel is of course necessary.