So that's now three links to the same page and three different distances the car should be from the wall don'tcha just love the internet
Meanwhile my book of complete car repair and maintenance (from 1988) suggests that the car should be between 5-10 metres from the wall
We did it by dead reckoning, if it looks right, it probably is. We parked it about half a car length from the wall and it didn't fail. No idea what the distance should be.
The distance from the wall can be any distance, but the dimension to the left of and below the marks will increase with increased separation of car and wall. The further away you are, the more accurate you are likely to be, but the test has a range of acceptance.