I am not real fond of the method they show because of the lump where the final fold is when you have large pieces of fabric.
I use the acrylic rulers as a guide, but with a twist.
I have several folds that I use. For fabric 1 yard or less, I fold it selvage to selvage and then in half again selvage to fold and wrap it around a 4" wide ruler. For over 1 yard to 2 yards, I fold it in half selvege to selvege, fold it in half selvage to fold and wrap it around a 6" ruler.
For over 2 yards I either fold it selvage to selvage and wrap it around an 8x24 ruler and store it in my deep linen closet, or I fold selvage to selvage and selvage to fold and wrap that around an 8" wide ruler.
This way I can see at a glance how much of a particular piece of fabric I have. It took a while to do it, but it is a great help now. Speak of the devil -- I have to go fold fabric and clean the sewing room right now. :D