Merriam-Webster Dictionary states that farther and further are interchangeable. However, recently, their use is becoming distinguished by grammarians. Here is the difference:
Farther should be used to show a relation to physical distance: “The downspout should discharge farther from the house.”
A little trick is to see if an actual distance can be used in place of the word farther in your sentence: “The downspout should discharge 6 feet from the house.” If it can, you were correct in using farther.
Further should be used when referring to degree, time, space, or extent or depth, as in: “This problem should be investigated further by a roofing contractor.”