In reality that line isn't going to help you much. You can't use it to guage with any accuracy BUT you can use it to do a quick visual check. If you are experienced enough you might be able to tell if you solution looks off. But all in all I barely ever look at it.
In fact since installing this mod:
http://www.subsim.com/radioroom/showthread.php?p=463988#post463988
I've not even pressed F6.
