Anchell's recommendations can be confusing because he's talking about both the minimum amount to cover the film and the minimum amount of stock to develop the film in the same context. Further, he's generalizing, which can be problematic. His recommendations in my opinion are intended to give you a huge safety margin.

The subject of developer re-use is a different matter. Here again it is dangerous to generalize because different developers (and the films are also factors) have different capacities and also respond to the buildup of development by-products, bromides etc. in different ways. This is one of the reasons why certain developers need specially designed replishers while a few others can function as their own replenishers. Specifically in the case of D-23 (Metol-Sulfite), I would not recommend re-using the stock solution if your goal is consistency. For one thing, as bromides and acidic development by-products accumulate, you will tend to lose film speed, and image characteristics may also change.

When it comes to some other developers, different people will say different things. For example, since Phenidone (and derivatives) are less sensitive to bromide, used XTOL (which contains no restrainer) can be replenished with fresh XTOL, and many people like to use it that way (undiluted of course). Kodak reminds us replenishment and/or re-use are compromises. But some people still claim replenished XTOL is "better". Just an example. There are also more exotic/older developers that were purposely "seasoned" before use.

My advice (only my opinion - but one consistent with the methods of many great photographers - and consistent with recommendations by Kodak and Ilford) is to not re-use developer, whether undiluted or diluted. If you wish to re-use undiluted developers such as D-23 I guess I'd suggest adding its replenisher to your workflow.