I don't have FM, but when I went for my last test after being on Iodine for 7 months, my MD (who knows I don't take prescription drugs) told me to take 1000 IUs a day.
I was surprised, especially since I know being out in the sun is a great help. Since 3 years we spend our winters in FL and had joined a nudist resort, so I figured I got more sun than the average person, but it seems that was not enough.