The focal length of the plano-convex lens is $1/f_l = (n-1)/R$. The focal length of the plane mirror is $f_m = \infty$. The equivalent focal length $1/F = 2/f_l + 1/f_m = 2(n-1)/R$. The equivalent mirror has radius $R_{eq} = 2F = R/(n-1)$.