This is impossible to say, earlier I have recommended 100 mcg of selenium per 50 mg of iodine.
This is what I have given kids.
Later I learned that selenium has a long half time in the body, it may accumulate. So if you have been on 600 mcg for a while, I would reduce a little; to like 200-400 mcg.
But always I would check the progress, symptoms I have, wellbeing. In general I dont change a winning team.