Maximal polarization degree P_max as a function of the magnetic field stretching factor ξ, in which synchrotron radiation is emitted. This relation is plotted for four combinations of the jet energy angular profile power-law index a and normalized viewing angle q, and is well described by a hyperbolic tangent of log₁₀ξ for all values. The region of ξ imposed by the upper limits on the polarization of GW 170817 (0.75 < ξ < 1.15; Gill & Granot 2018, 2020; B24) is marked in shaded gray. The inset shows a zoomed-in image of this region. All curves change sign at ξ = 1. Detailed models can be found in Table A.1.