This note aims at providing evidence that there exists a flow-based scaling for the Gurney flap heights that yield an increase in lift-to-drag performance compared with the baseline airfoil at the same angle of attack (beneficial Gurney flaps). The results presented here, support this statement and further suggest that the boundary-layer thickness δ, measured at the trailing edge on the pressure side of the baseline airfoil, is not only an appropriate flow-based normalization for the flap height but is also a proper order of magnitude for the flap height providing the largest increase in the lift-to-drag ratio (optimum Gurney flap)