Isn't a refractometer only effective for getting OG? The Beer Bug is designed to monitor constantly during fermentation.
You can use a refract to get FG you just need to apply some correction factors to the reading. This is because, If I recall, the refractive index of sugar and alcahol are similar so the refractometer can't distinguish when alcahol and sugare are both present.
Sean Terril has a good calculator for this on his site.