When you are developing, pour X ml out of A where X is the minimum amount for your tank and rolls of film
When you use replenishment, you no longer need worry about minimum amounts. I fill my 1 litre Paterson tank with one litre of developer, no matter how many or how few rolls I'm developing. As almost all of the developer goes back into the bottle for re-use, there is no concern about wasting developer, and I never have to worry about not covering the reels.
Where this really shines is where you are developing sheet films, where some of the tanks take several litres of liquid, even if you are only developing one sheet.
One caveat - while I have used and recommend X-Tol replenished, I/m actually currently using HC-110 replenished with the now long discontinued special purpose HC-110 replenisher. I've fine tuned that combination to my liking, but it is a fair bit more finicky than X-Tol to use replenished, so I recommend X-Tol (or T-Max RS) for anyone starting replenishment now. Special purpose replenishers are, relatively speaking, much more of a bother to use.