That's one of the shortcomings of this attachment - you do need to use some scrap strips to figure out how much fabric the ruffles will need.
Not only does the ruffler behave differently on different fabrics, so that you get 30" of ruffles out of 42" of batiste and 27" of ruffles out of 42" of a Moda-like quilting cotton when the ruffler is set on 12, (a tuck every 12 inches) there is also a setting to determine how deep you want the tucks to be.
And if that's not enough variability for you, the tension and stitch-length settings on your machine affect the finished product, too.
So, buy extra fabric for experimenting and figure in a pretty good "fudge factor!" :)