Could you not also tie it into if the ceiling fan is running? Or a flow sensor on the shower?

I’m just thinking of natural humidity and how that might skew the numbers.

That’s a really cool and neat way to pull it off. I helped myself shower more by attaching a NFC tag to the shower wall and have an alarm my phone that can only be turned off by tapping the tag. The thinking is that I might as well shower while I’m there.