“Farthest” vs. “furthest”

Question

My spellchecker insists on replacing "furthest" with "farthest". I was under the impression that farthest is strictly speaking in terms of distance, whereas furthest is more abstract.

A poster on this forum gives the following example:

The further [quantity] you walk, the farther [distance] you get.

What gives?

Answer 1

The NOAD reports the following note about those words.

Traditionally, farther and farthest were used in referring to physical distance: the falls were still two or three miles farther up the path. Further and furthest were restricted to figurative or abstract senses: we decided to consider the matter further. Although farther and farthest are still restricted to measurable distances, further and furthest are now common in both senses: put those plants the furthest from the window.