The angle of a draft feature is measured against the normal of a draft plane you define. Ideally, the draft plane should be perpendicular to the faces that you want to draft.

For example, to add a draft angle to the four vertical faces (A), you can select the top face (B) as the draft plane, since this face is perpendicular to the faces you want to draft.

For a simple drafted feature, the faces you want to draft are pivoted about the draft plane. For more complex drafted features, the faces you want to draft can be pivoted about a part edge, parting line, or parting surface.