It is Always an estimate depending on the tilt of the road and other angles. Maybe it Can be more accurate than with other cars since the tesla has à bunch of sensors. But usually, each gradation is 50cm more than the previous one, depending on the model.
My old Prius and current Lexus have X-shaped lines to guide you into parking spaces (and around corners) that I find infinitely more useful than the two parallel lines option, which I've never found a use for.
Many cars have the guidelines curve to show the actual curved path as the steering wheel is turned, this is much more helpful than always-straight lines. It makes parking back-in a snap, as we often find ourselves doing at charging stations.
I don't use the backup camera when parking until the very end because I use the side mirrors to accomplish this goal. I would much prefer to use the guidelines on the single screen, since I will park in places not always painted with stripes.
My LEAF has this, and they turn with the steering wheel to show you where the car will go. The software can't be hard as long as the hardware has the ability to overlay CGI on top of the video from the camera.