Hi Colleen,
I could give you a technical reason for why you cross-multiply but that wouldn't do much for the "understanding" part of the lesson. I had a few minutes before we have to leave for the infusion center and I found these two web sites that may shed some light on what you want.
www.eduplace.com/math/mathsteps/6/a/index.html
And www.homeschoolmath.net/teaching/proportions.php
The second one is pretty good. It uses tables a lot to develop the proportional thinking. I couldn't find anything as of yet to do with the line segment model. Will look later when we get home but it will be late so I don't know how much more help I'll be. Good luck.
Ron

< replied-to message removed by QT >