The use of Doppler Backscattering (DBS) in spherical tokamaks is challenging since the magnetic pitch angle can be large (up to 35, compared to 15 in standard tokamaks like JET). Moreover the pitch angle varies both spatially and temporally. Hence, the probe beam is generally not perpendicular to the magnetic field. This misalignment, which affects the backscattered signal, can be empirically optimised with 2D beam steering . However, empirical optimisation is inefficient, requiring repeated pulses with different diagnostic settings, and may not always be possible. Hence, it is important to develop a model to quantitatively account for the effect of the misalignment on the backscattered signal, avoiding the need to optimise empirically. We use beam tracing (Torbeam  as well as a newly written code) and the reciprocity theorem  to derive a model for the backscattered power and its dependence on the mismatch angle. Our model works for both the O-mode and X-mode in tokamak geometry.