I have found that the 45mm cutter works best for me with the Shape Cut Ruler. I think the smaller one probably isn't getting all the way down to the cutting board between the thickness of the ruler and fabric.
Is it the width of the border or the length that is off? I think 1/8" isn't too much to ease most times. Depending on the fabric, you might be able to piece it together in a way that won't be too noticeable.