Left panel: ejecta mass distribution as a function of age and density assuming equality in Eq. (4). For each point of the grid, the swept-up mass in the equation is derived by computing the shock radius using evolution equations for a supernova explosion energy of 10⁵¹ erg, n = 9, and s = 2. The white region indicates the excluded parameter space, where the ejecta mass becomes negative. Middle panel: using the ejecta distribution previously derived, the same explosion energy, and ejecta and density profile indices, the ratio R_sh/V_sh is obtained and shown here as the distance independent ratio . The value of this ratio measured in X-rays is shown as a solid white contour (dashed contours for statistical errors). Right panel: for a fixed age of 2250 yr derived from the previous panel, the radius of the SNR (or its distance for a fixed angular size) is explored as a function of the explosion energy and the wind density. The solid white line represents the radius of the SNR at a distance of 1 kpc.