To do this, choose the dimension menu item, click on the middle of the line with the left mouse button, then on the datum plane with the left mouse button, then with the middle button where you want the dimension to appear. This will then appear as an angular dimension which can then be modified. Always align centerlines to datum planes, or explicitly dimension them at a certain distance or angle away. This creates two separate aligns for that point. Center points of circles need to be aligned or dimensioned.
P is the number of points on the sketch.
Yes, you probably don't want another equation. You may not even use this equation, but going through how it was derived could show you a better mental model for figuring out how many dimensions and aligns you need. Think of a point lying on a plane. Hence the 2P. Think of a straight line at some arbitrary angle. Think of a line parallel to a datum plane.