Normally you plot abs(Gain) at y-Axis, so you can calculate the phase margin like an noninverted opamp. This means you start at 0 degree.
Or you can handle your problem as in your attempt, but this isn't industry standard.
While speaking about gain and phase, it is important that you know about WHICH amplifier properties you are speaking: And if you are interested in the stability margins you have to investigate the LOOP GAIN only (in contrast to the closed-loop gain).
Your drawing is correct because you have opened the loop at the inverting input terminal (this introduces no significant errors because the opamps input resistance is very large.)
Then. the loop gain is the ratio of the voltages between both resistors (output of the loop) and the voltage at the inv. terminal (loop input).
If you plot the gain vs. frequency and the phase vs. frequency it is very easy to apply the stability criterion.
EDIT: Be sure to perform an ac analysis over a suitable large frequency range (not a TRAN analysis as indicated in the drawing).