I think the definition of "excess" depends on the individual but can be identified based on the level in the blood and urine. That is why it is hard to establish a totally "safe" level that is also therapeutic for everyone.

I am not a physician, but I ran large physician groups of over 400+ PCPs and specialists. I was made aware of the dangers of too much fluoride by them.

As mentioned, the issue is twofold. Some individuals get exposed to fluoride through many other products they use daily and so the introduction of an additional source, many not be beneficial.

Secondly, some individuals have to restrict their fluoride intake for health reason like kidneys unable to excrete excess fluoride and so adding more fluoride to their consumption intentionally could be problematic.

You can browse the internet for studies as well as read those referenced in the links below, but while there may be some evidence to how much fluoride a healthy person can tolerate (more being deemed "excess"). I would think the bigger issue is how much can someone who must restrict their fluoride can tolerate.

https://pubmed.ncbi.nlm.nih.gov/34597567/

https://pubmed.ncbi.nlm.nih.gov/30472891/