The author makes a good point, but the example code was a little terse. The basic idea is to follow common sense and not reimplement the same tedious code every time you subclass. Instead, find a common point where you can implement a hook in a subclass that performs the critical change.

Usually, I prefer the Action delegate pattern, or whatever it's called. For example, I would instead give the calculation routine an Action delegate that performs the critical calcuation. So, instead of making a whole new subclass, I'd have individual delegates premade for the different calculations I'd need, or roll my own if I had to.