This is dependant on the catenary wire height. It's easy enough to set when you have a catenary spline point over a track spline point as they should both be the same height.
 
The problems start when you start to add additional catenary splines to ensure the wire remains about the centre of the track. This involves manually setting every height and the only way to check they are spot on is to switch to driver and run a train along the section. I asked if anyone was willing to make a simple tool that would measure the height but so far, I've had no response.