Although the gauge for this pattern is given for a swatch done in  Stockinette stitch, you must take into consideration that all yarns of the same gauge do not “ruche” the same. Your final measurement once the fabric is ruched will depend on the elasticity of the yarn and your knitting tension, i.e. how tightly (or loosely) you knit. With that being said, in order to get an accurate swatch measurement you should do a ruched swatch as follows: