Further v. Farther

In going through a letter I wrote recently, I found this example of where further and farther are not interchangeable:

"Your failure to do so will give our client no option but to take further legal action as we deem necessary to recover the damages occasioned by the faulty construction provided by ABC, LLC."

However, I did read the rule concerning further and farther again in the Chicago Manual of Style, and it describes the distinction as "the traditional distinction." CMS 5.202. While it doesn't say that the distinction is no longer used, it does indicate that the general distinction is not necessarily the rule.