On this occasion it was quite easy, Google told me the height of the cooling towers so I added a bit on to miss them.
The difficulty comes with tress and things that are not easily judged from a 2D map.
Others have used the Google Earth method which works well and allows you to fly a route virtually before you go out
As for judging distance and height, thats just a matter of practice and something you may never get better at, imagine Mrs Geek driving 