Politely, I beg to differ. As you and I are both jet fixers, consider my words. If, the Spyder has toe out prior to alignment, the laser point projected to the square will not be the accurate track width, plus laser and fixture. Add to this, the machinist square uses a small base, that amplifies error as the square may not be truly square on the floor.
The ROLO setup, uses a laser fixtures that swivels to project downward, eliminating the error induced via incoming toe settings, and the machinist square transferring a point to the floor.
Both methods are working to obtain the same reference datum points.
Your setup uses pie plates on stanchions. If the stanchions are each exactly the same, the pie plate runs true in plain without runout if the wheel could be spun. (The brake caliper prevents this). Possibly your setup could point the beam down, similar to ROLO, and would allow you to compare the two methods for accuracy. Downside to this being, if you have already established accurate toe setting, that error is eliminated or reduced. Whereas, the swiveling setup has no care of incoming toe setting.