Simplified parameter optimization of our kinematic model. The grey scales represent the similarity of model and observed HDO triple-peaked line profile (lower values for better match). The solid curves with values in unit of km s^-1 arcsec^-1 indicate the measured strength of linear velocity gradient at PA 0° in the grid models. The open circle highlights our best matching parameter set (v_R0 = 10 km s^-1 arcsec^-1, ξ = 0.35).