quartz crystal - figure of merit
Your equation for figure of merit M, AKA the Q of the circuit, is correct as long as you recognize that \$f\$ is the resonant frequency, and \$C\$ is the capacitance that sets the resonant frequency.  In this case the resonant frequency is set by the motational capacitance and inductance, \$C_1\$ and \$L_1\$.

So \$\omega = \frac{1}{\sqrt{C_1 L_1}}\$, and \$Q = \frac{1}{\omega C_1 R_1}\$.