Having a bit of trouble with figuring out my attempted tweak. I know it's not possible to "crop" a 3D model in the traditional sense, so what I'm trying to do is simply print only the layers I want. In my brain anyway, it should be fairly simple. Here's the model:

http://www.thingiverse.com/thing:239792

I basically just want to print the top section. I mocked it up in my software, and it would come out to around 650 layers using my settings. Is there a tweak in the gcode or elsewhere to where I could print, for example, layers 450-650 to save all that other space?