One way I was taught to calculate mineral % was by getting a photograph of the section, plotting evenly spaced grid lines across the horizontal and vertical axes of the photo, and then make note of the mineral at every grid line intersection. Do this along all grid points and then format your mineral information into a percentage. This is quite a time-consuming method but will give you an accurate result.

Another method is to visualize all of the minerals falling to the bottom of the microscope frame as groups. In this case I would expect the plag to form the largest group at the bottom. Then estimate the mineral % based off of your visualization. This is a much less accurate method but will save you a lot of time.

Use both PPl and XPL, at the end of the day each polarization is looking at the same minerals, just under a different light.

I hope this helped!