>The key quantity on which the argument hinges is

    T * c / D

I'd think that it's also important to have a component that quantifies the lifetime of a technologically advanced civilization, as the universe isn't just separated by space, but also by vast expanses of time as well.

I'm not belittling your example, just trying pointing out that when speculating in this area, it's useful to include a time component related to civilizations as well as an individual lifetime component.