A fluid flow control adjuster device controls fluid flow ratios when placed in-line with a tube that runs from a toilet fill valve to the overflow tube of a flush valve. The device comprises an adjuster having an adjuster body with an inlet and an outlet. The body includes an adjuster knob and a receiver gap, which is a gap between a body rotation stop and another edge at a top surface of the adjuster knob. A key is also provided which can be inserted between the body rotation stop and the edge of the adjuster so that the installer can dial in the correct percentage of fill. The key is a hub-like planar structure that matches the curvature of the receiver and includes a plurality of key fingers with each finger including indicia that correlate to settings for selecting a proper toilet bowl refill range.