View from above and set your corner vertices about where you want them. Then click 'Split Spline' (lower right button), and then click on the corner vertice, viola. Hold the 'Shift key down and zoom in (down close) to fine tune your corners. This allows post corners to merge if yer really fussy. May have to move one out and away first to square it. Just keep the 'Shift key' down while arranging them with the mouse. Should work for all splines.