I think right now, the documented "safe upper limit" is 400mcg. In addition, I've seen multiple physician websites recommending Selenium supp at 200mcg daily and "not to exceed".
It depends where you look. The NIH
for example say 400 mcg is the daily upper limit.
Safe dose level is an academic discussion, however, unless high-dose selenium actually improves your ME/CFS. Not many ME/CFS patients benefited from this high-dose protocol unfortunately, and if you don't benefit, the max safe dose is not something you have to concern yourself with.
In my case, 400 mcg of selenium daily brought me from moderate to severe ME/CFS, to moderate to mild. So I would rate that as a ½-level improvement on the ME/CFS scale of mild, moderate, severe
I do not want to go back to bordering on severe ME/CFS. As soon as I stop selenium, I start getting worse again with 5 days. I have tried stopping several times, and getting worse is always the result.
So whatever the risks of high dose selenium (and I don't think there are any), those have to be balanced against the substantial improvement I have made.