3/4 off their shoulder?

[Eway]

"Drafting is running behind someone, 3/4 off their shoulder, and it can save up to seven per cent of your race energy output by minimizing your air friction."

Does "3/4 off their shoulder" mean that keep a distance of 3/4 shoulder length? (Isn't this too close? Wouldn't the runner tirp over each other?)

[svartnik]

"Does "3/4 off their shoulder" mean that keep a distance of 3/4 shoulder length?"
Yes.

[Eway]

But...a latter part of this book says:

"Find a racer who is only slightly faster than you, and fall in just behind his or her shoulder. Maintain a courtesy distance of about 1.5 metres...."

How can a person's shoulder as wide as about 1 metre? What does this 3/4 refer to?