Parameter space of the model grids for 3C 285 (left) and NGC 541 (right). The axes indicate the value of the outer d₁ and inner d₂ distances of the Toomre ring (see sketch in Fig. 8). Since d₁>d₂, the explored parameters lie below the blue line (bottom-right part of the plot). The black solid lines show solutions for a peak velocity of 160 and 175 km s^-1 (left), and 90 and 110 km s^-1 (right). The black dashed lines show the solutions for a well depth (see Fig. 10) of 30% and 35% of the peak (left), and 25% and 30% of the peak (right). These values correspond to the observational constraints extracted from the 30 m telescope spectra. The dark blue surface is the intersection that fits both solutions.