In Sec. 9, we have provided the steps to construct the magnetic surface coordinate system (ψ,𝜃,ϕ). Only one additional step is needed to construct the straight-line flux coordinate system (ψ,𝜃,ζ). The additional step is to calculate the generalized toroidal angle ζ according to Eq. (250), where δ is obtained from Eq. (255). Also note that the Jacobian of (ψ,𝜃,ζ) coordinates happens to be equal to that of (ψ,𝜃,ϕ) coordinates.