Is it possible to plot angles for the pie chart using a calculated field?

Yes, you can do so!

just take a normal example where you need to show category vs sales. no, do some reason you have to subtract 10 from each sale. To achieve this you will be definitely creating a calculated field. And as you can see this calculated field is nothing but a little transformed version of your original sales measure. And so, you can use this calculated field in the pie chart to plot the angle.