Here is a link with some comments on further vs. farther. There are some fine distinctions if you want to make fine distinctions, but to me they are nitpicking. I am comfortable considering them essentially interchangeable.

http://www.andrew.cmu.edu/course/76-451/watts.html


>I don't know if this is what you're saying but I've always taken "farther" to refer only to distance in space, and "further" for other things, like degree of change, as you've put.
>
>OTOH, I've only ever see farther used much in American lit. and hardly ever used by a Brit. and, probably therefore, a kiwi or oz. We'd always say "further" for all pf them.
>
>>Further is more distant in degree, time, or space than someplace or something else.
>>
>>Usually used not for distance itself but in:
>>
>>further from the truth
>>don't discuss it further
>>he is further along in his studies than she is
>>
>>It is often used in place of farther though in vernacular speech...
>>
>>
>>
>>>>Is it further than Motueka (New Zealand) ???
>>>
>>>Not sure I got your joke. Did you mean to say "farther"?