As I age, I realize I can't handle as much hot water as I used to. But a hot soak that isn't hot, isn't a hot soak. So, I found the perfect temperature and marked the spot with a pretty gem you can barely see. As I am in a rental, I will be able to take it off when I leave. So I used a generic E-6000 glue, from the Dollar Tree, and let it dry overnight.
It looks nice, helps me to save water by not having to waste some down the drain, if it's too cold, so I can fill it with hotter water. This is my Goldilocks spot, just right.
Hope that helps!