  It was shown in [Augusiak et al.,\;Phys. Rev. A \textbf{77}, 030301(R)
(2008)] that discrimination between entanglement and separability in a two
qubit state can be achieved by a measurement of a single observable on four
copies of it. Moreover, a pseudo entanglement monotone $\pi$ was proposed to
quantify entanglement in such states. The main goal of the present paper is to
show that close relationship between $\pi$ and concurrence reported there is a
result of sharing the same underlying construction of a spin flipped matrix. We
also show that monogamy of entanglement can be rephrased in terms of $\pi$ and
prove the factorization law for $\pi$.
