Solution
= Solution
For the homogeneous background, $X=\dot{\bar\phi}^{,2}/2$. The second <Friedmann equation> gives
$$
\frac12P_{,X}\dot{\bar\phi}^{,2}
=-\dot H M_{\rm Pl}^2
=\epsilon H^2M_{\rm Pl}^2.
$$
Dividing by $H M_{\rm Pl}^2\dot{\bar\phi}$ shows that the coefficient found above is
$$
\boxed{C=\frac{H\epsilon}{\dot{\bar\phi}}}.
$$