This paper describes a parametric design methodology for maximizing the capture factor (CF) of a bottom-hinged flap-type wave energy converter (BHF-WEC). The general equation for CF is first derived using the damped-harmonic-oscillator model. Second, correspondences between the general and the 2-D ideal CF equations are established. Then, a scheme is proposed to account for any effects apart from the 2-D ideal modeling with three parameters, which constitute the basis for fitting any data series stemming from either numerical simulations or experiments. Once these three parameters are evaluated from data fitting, the maximum CF and its occurring conditions can be found. In the present study, WEC-Sim simulations are conducted for a series of finite rectangular BHF-WECs with effects of PTO and varying width (B) for two thicknesses (d) under two characteristic wave lengths (L) of the medium wave resources that Taiwan possesses. It is found that for B/L smaller than about 0.30, the maximum CF in resonance mode, CFres, is greater than 1.0 and much higher than that not in resonance mode, CFopt, which is always below 1.0. The captured power index in resonance mode, CFres × (B/L), is almost invariant in B/L = 0.11–0.30. Several BHF-WEC design guidelines can be deduced from these results.