Capacity is mainly limited by the amount of active chemistry available for a given film area, not by water alone. In other words, the film consumes developer activity; water mostly provides volume and even coverage.

That’s why manufacturers often specify a minimum amount of stock developer per roll. One answer cites Kodak XTOL: one 35mm-36 or 120 roll needs about 100 ml of stock developer regardless of dilution. If you dilute it more to reach the volume needed to cover the film, you may still have enough chemistry, but development time usually must increase because the active ingredients are less concentrated.

So rotary processing can “save chemicals” only because the tank design lets you use a smaller working-solution volume while still maintaining the required minimum amount of active developer. If you go below that minimum, capacity drops and results may become inconsistent.

In practice:

• total film area processed determines exhaustion,
• minimum stock/concentrate per roll must be met,
• extra water does not increase capacity,
• larger volume may be required simply to physically cover the film in non-rotary processing.

So yes: 240 ml vs 485 ml can matter for convenience and economy, but only if the smaller volume still contains enough active chemistry for the film being processed.