I made close to the same type thing and used for a while, but just simpler to look up front and rear track widths for mental note and comparison, then assume rear axle on RWD is square and then shoot a line to front tires and compare as the line shot passes front tire where it is. Any thing within a 1/16" will work fine, all that computer crap on alignment machine spouting this side off by .1 degree is just that, crap. The rubber parts alone allow for deflection of 1/16"+ from simply sitting still to dynamic and moving. Meaning all the online arguments about this or that setting varying by 1/10 degree are usually bullsh-t.
A little more complex on FWD but still easily doable, I've done 200mph race cars that stayed dead straight doing it, no troubles at all. A laser would make it a bit easier but I haven't seen enough issue to get one yet.
Front to rear tracking (thrust) can be checked pretty easy by finding say a park, dead flat parking lot and run car a few feet to line out straight with steering wheel where it stays on freeway at speed and when lined out then cross a slight water puddle after rain with all four wheels, roll out to dry tires and then go back and look at the front to rear wheel tracking marks laid down by tires. One set should be pretty much centered on the other. A great free tool.