Agree with the Hydrometer reading. The most important ingredient with wine / mead musts is time. If the fermentation slows down, take a small sample and test with a hydrometer. At this stage, take a taste to see if it is at the desired sweetness. If so, you can stop the fermentation using...
Why not just back sweeten with honey to desired taste then pasteurize to ensure that no further fermentation occurs?
That is providing that you are happy with a 13% mead of course.