Think about how much time you’ll save not having to pee in a measuring cup before dumping it in a toilet! So much time to gather other metrics about your life
It would be off the waterline of the individual toilet. I imagine the level indicators would need to vary from toilet to toilet because of different toilet bowl shapes.
This could create a bridge between those companies making fly stickers for urinals and the big toilet corporations.
The water line in the toilet stays at the high water level of the trap, it doesn’t fill as you pee. Back to the measuring cup!