Re-define central optics cut-out.

Define the cut-out from the inner wall positions, not from the not from back_lug_x_pos().

Together with !429 (merged), this MR means that the inner wall and the central cut-out behave as expected when the stage size (leg_r) or stage height are changed.

Small correction factors in both !429 (merged) and here make the actual lug position the same as in previous versions, and the cut-out shape very nearly the same as in previous versions when using the standard stage parameters.

Related discussion in #394 (closed) from #393 (closed) and !429 (merged):

closes #394 (closed)