All rafters have a DP- Shadow Line. When the rafter is plumb, the DP- Shadow Line is the same as the rafter run line. The Hip Rafter Trace Angle is developed from the intersection of the DP- Shadow Line of the two rafters and a line that is perpendicular to the DP- Shadow Line of the hip rafter, that intersects the rafter run line at the hip rafter. In the drawing below the hip rafter is canted. The Hip Rafter Trace Angle on the canted hip rafter is the same angle that can be used on the Witches cut. The purlin rafter in the drawing is rotated into the roof surface at the same angle that is used for the rectangle rafter tails of the Witches cut.
In this drawing I tested the theory on a plumb hip rafter and yes it works correctly on plumb hip rafters.
In the drawing below the roof slope is unequal and the hip rafter is plumb. Using this drawing technique it makes it pretty easy to draw out the Hip Rafter Trace Angles for the Witches cut or hip rafter purlin housing angle.