Farther vs further

I am a stickler for correctness and old-fashioned, stick-in-the mud, grammatical nitpicker which I often try to mend, to no avail. It is difficult to teach a very old dog new versatile tricks!
In my book, farther means beyond a distant point, the comparative of far. New York is far, Tokyo is farther. I use farther only for physical distance.
And in my book, further, means moreover, additional: No further comment.
I think I will continue like this because I like to sound quaint, bizarre and olde worlde.  But farther and further have been interchangeable since the beginning of time. So, feel free to say that Tokyo is further than Scranton, Pennsylvania.