That method can be perfect enough to never need to pay for alignments as I do. I line up my own using that and other ideas. Once you are close enough to start splitting sixteenths you are there, no car on the planet cares about amounts of error smaller than that, the deflection from actually driving the car will be more. It's funny to hear all these people talk about digital numbers that bye and large mean very little in many cases.
The rear may be SUPPOSED to be narrower than the front but careful measurement can show whether that too is the case real world. Every car can vary slightly from that, you just gotta shoot the lines and see where they end up, then adjust to fit the need.