Am I using more hair dye then I need to?

I've been dyeing my hair for almost two years and I've noticed that I seem to be running out of dye more quickly then I used to. I currently have hair dye on my head right now and have used three bottles of ion's dye and it still hasn't covered all of my head, so I'll need to use a fourth bottle! This seems ridiculous to me. I used to use two bottles when I was starting out and besides some missed spots that seemed to be enough. That was when I had longer hair too! But here I am with medium length hair, going on four bottles. I don't know if it's just because I'm a bit more thorough now, or if I'm misjudging whether a spot needs more dye or not. I'll put a picture of myself below so you can get an idea as to the length of my hair. Am I using too much? If so do you have any tips/techniques so I can avoid using so much hair dye? Because this seems ridiculous.

picture: http://i.imgur.com/R3ULAqp.jpg