'Farther' is sometimes used for physical distances, while 'further' is often used for figurative or metaphorical distances, although this is not an absolute rule.
Also, 'further' can have a meaning of 'additional' (as in your example sentence) that 'farther' doesn't.
And even then "further" works just as well. I don't see any need for "farther". A 15th century spelling mistake.;-)
farther (adv.)15c. alteration of Middle English ferther (c. 1300), a variant of further (adv.). There is no historical basis for the notion that farther is of physical distance and further of degree or quality.
Online Etymology Dictionary.
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: