Steve, I have used the "cut flex here" command. Basically, you add a piece of flex to the layout, then position the pointer over the location you want to cut, and it cuts the flex into two connected pieces.
You can also work with flex pieces after added to the layout: you can select the end of a flex piece and lengthen or shorten the piece with the mouse. Hard to describe better than that, just try it if you have not already.
