Node Manipulation Question

Hello all,

Quick question for you experts. Is there an easy to constrain the movement of a node at a particular angle / trajectory? I have an existing arbitrary angle that I would like to lengthen. I was hoping that if I hold the shift key, that it would constrain the line to the current angle / trajectory, but it's not working (it snaps at 15 degree increments from zero degrees). So, does anyone know how to constrain a node / line to the existing angle of a line? See attached  example. Thanks!

Mike