It is quite quick and easy to make a contour symmetrical by using the mirror tab on the Transformation Toolbar.
Here is how I made sure the circumflex accent was perfectly symmetrical.
Stage One
Select two nodes and split the contour
Stage Two
Duplicate the left half, mirror it horizontally, repositioning it using guidelines for accuracy. Then select the top two nodes to be joined.
Stage Three
Right-click on one of the selected nodes and join them.
Finally, delete the one extra node from bottom centre.