It should be obvious that at least this is true; when arm angle = 90, the ends of the arms do not move in or out as the arms move, so that pulling the cable 1 mm moves the arm up 1 mm no matter the yoke angle.

(it may be easier to determine MA in terms of "how far does the output move when I incermentally move the input" rather than "how much force comes out when I put a certain force in." Both methods have to give the same answer but I find the first to be easier to think about when I'm drawing diagrams.)