What is the the correct technique when measuring the distance of a result outside of the viewport?

Measure from the outer edge of the viewport is the correct technique when measuring the distance of a result outside of the viewport

Explanation:

To obtain the correct distance of the result that is generated outside the viewport, it is preferable to measure the distance right back to the out of the viewport.

This is because, the outer edge of the viewport is where the viewport ultimately begins from.  Whereas, measurement from centre and to user's location are not best way to calculate distance.