Okay, this image definitely looks confusing at first, because there's two things happening here. First, I took the measurement of that square in 3D space and carried it down four more times to establish five equal squares. Remember that the car is roughly 6 wheels long? Well, in observing my reference, I also noted that there are three wheel-lengths between the front and back wheels. This leaves roughly one wheel to be split up between the section in front of the front wheel, and behind the back wheel. To my eye, the front looks to be about 1/3 of a wheel, and the back appears to be 2/3. So what do we do with this information?

First, I don't want to clutter up my drawing by first measuring out six wheels, then placing the actual wheels in a way that doesn't actually align with any of the wheels I've put down in order to measure things out. So, I drew five wheel lengths, with the first and last ones actually corresponding with the car's actual wheels. All that's left is to tack on the front and back sections. But how?

That's where the additional marks come in - first I eyeballed a point that is about 1/3 of the way into the front wheel. Then I mirrored this measurement up and across the left side of that wheel's plane, giving me that same measurement directly to the left of the wheel. Did the same thing for the back - eyeballed a point 2/3 of the way into the back wheel, then reflected it towards the right.

These transferring techniques are extremely useful when trying to figure out little finicky things like this, while maintaining a degree of accuracy.