  We show that pressure applied to twisted WSe$_2$ can enhance the many-body
gap and region of stability of a fractional Chern insulator at filling $\nu =
1/3$. Our results are based on exact diagonalization of a continuum model,
whose pressure-dependence is obtained through {\it ab initio} methods. We
interpret our results in terms of a {\it magic line} in the pressure-{\it
vs}-twist angle phase diagram: along the magic line, the bandwidth of the
topmost moir\'e valence band is minimized while simultaneously its quantum
geometry nearly resembles that of an ideal Chern band. We expect our results to
generalize to other twisted transition metal dichalcogenide homobilayers.
