I want to eventually extend my "z" from 3" of travel to, 6" of travel. So how long will the lead screw need to be? I can't quite wrap my brain around this obvious question. Not including the motor, what I have now for a lead screw is approx. 8" long. If that length is giving a 3" travel should an additional 3" be added to get 6" of travel, or will I need more. I had originally thought that a 12" screw would be sufficient to yield a 6" travel, but now I'm not so sure. Comments are welcome. This is what my machine looks like