How to draw a perfect arc in Keynote

To draw a perfect quarter circle in Keynote (the OSX presentation package)

1) Work at 400% (or whatever for greater accuracy) 

2) Draw square first

3) Choose Insert|Shape|Draw with pen (to start drawing a Bezier curve)

4) Click on first point (e.g. bottom left corner of square)

5) With mouse still down, drag the tangent upwards to next corner of square (e.g. top left)

6) Release mouse

[That's the first point done]

7) Click mouse at end point of arc (e.g. top, right corner of square), release. 

8) Press escape key

Hey presto, you should have a perfect quarter circle.

You can now delete your square

Then you can go on to treat your arc as a 'line' and adjust the thickness, colour, etc