spectre stb analysis
When your parametric simulation is finished plot the Gain-Phase curves. You'll have one curve for each value of the cap.
Select the phase curves with the "familly" function of the calculator, and use the
phaseMargin function to get thePM. When printing the result you should have a table of the PM versus the cap.
If you want to know more about opamp design with Cadence you can read the following tutorial. There are some scripts samples at the end for automation.
Good luck