= Solution
For intrinsically symmetric oppositely directed <relativistic jets>, their <proper motions> are
$$
\mu_{\rm app}=\frac{\beta c\sin\theta}{D(1-\beta\cos\theta)},\qquad
\mu_{\rm rec}=\frac{\beta c\sin\theta}{D(1+\beta\cos\theta)}.
$$
The ratio removes $D$ and yields
$$
x\equiv\beta\cos\theta=\frac{\mu_{\rm app}-\mu_{\rm rec}}{\mu_{\rm app}+\mu_{\rm rec}}=0.4.
$$
Taking the product instead gives
$$
\mu_{\rm app}\mu_{\rm rec}=\frac{c^2}{D^2}\frac{\beta^2-x^2}{1-x^2}.
$$
<Causality> requires $\beta\leq1$, so the limiting distance and inclination are
$$
\boxed{D_{\max}=\frac{c}{\sqrt{\mu_{\rm app}\mu_{\rm rec}}},\qquad
\theta_{\max}=\arccos x.}
$$
Here and in <3/e>, “maximum” denotes the limiting $\beta\to1$ bound; massive ejecta obey strict inequalities. The conversion is $1\ \mathrm{mas\,day^{-1}}=5.6113\times10^{-14}\ \mathrm{rad\,s^{-1}}$, hence
$$
\boxed{D_{\max}=3.435\ \mathrm{kpc},\qquad\theta_{\max}=66.42^\circ.}
$$
Both bounds occur together in the light-speed limit. At any specified smaller distance, $\beta^2=x^2+(1-x^2)(D/D_{\max})^2$ determines the intrinsic <speed>.
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