Described is a method for controlling fluid volume balance. A controller is configured with a first set of inputs comprising a hematocrit, a total blood volume, and an ACD ratio. A maximum extracorporeal RBC amount during the procedure is estimated based on the first set of inputs. A fluid circuit is primed with a priming fluid. Whole blood is drawn from a blood source and separated into a RBC component, a target cell component, and a plasma component. The target cell component is directed to a product container. The product container comprising the target cell component is treated. A treated target cell component, a portion of the RBC component remaining in the fluid circuit, and/or a portion of the plasma component remaining in the fluid circuit are returned to the blood source. A first response action is provided if the maximum extracorporeal RBC amount estimated is above a programmed limit.