= Solution
Use the same event pair already made simultaneous in the rear <instantaneous rest frame>. Its proper spatial separation is the <Minkowski interval> length, so
$$
\boxed{[h(t_B)-f(t_A)]^2-c^2(t_B-t_A)^2=L^2,\qquad
h(t_B)-f(t_A)>0.}
$$
The inequality selects the front rocket rather than the equally long backward extension. Equivalently the Lorentz-transformed spatial difference obeys $\gamma[h(t_B)-f(t_A)-v(t_B-t_A)]=L$. This is not the condition $h(t)-f(t)=L$ at equal laboratory times, which would describe the stretching arrangement in <4/a>.
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