When either works
For physical distance, “further” is also correct, especially in British English. The reverse is not true: “farther” for “more” (“farther details”) is wrong. If unsure, “further” is never wrong for distance; “farther” is only right for distance.
Catch it automatically
The grammar checker flags common mix-ups like this one by looking at the words around them, and the sentence checker can explain any sentence you’re unsure about.