How to determine position of clippingplane show

I’m working on application of 3D Molecular visualization. I need to display the angles of the atoms as well as their values.For the display of the angles I used the clippingplane which works perfectly. But I can’t position the corresponding texts correctly. I don’t know how to determine the coordinates of the center visible face of the clipping.

Please find my jsfiddle project :

Do you have an idea ?

Thx in advance