The amount of gather you need for an attractive fullness mainly depends on your choice of curtain heading tape. The main four are shown here, starting from the top left and going clockwise they are Eyelet, Pencil Pleat Pinch pleat and Goblet Pleat. The choice of curtain heading can radically alter the final look of your curtains and really should be thought about as you pick your curtain fabric
Pencil pleats are a more traditional curtain header. Here the amount of gather can be varied a little, however the curtain look f far nicer with the full possible gather of two and a half times the length of your curtain pole.
Pinch pleats can give a very sophisticated look to plain fabric, becoming the main feature of the curtain itself. This sort of pleating suits curtain tracking more than a curtain pole. Your curtains will need to be two and a quarter times the length of your curtain tracking for these pleats.
Goblet pleats will also give a curtain a regal feel. look. Again suitable for plainer fabrics on curtain tracking without where the pleats will be more easily seen. Goblet pleats also require your fabric to be 2 and a quarter times the width of your tracking.