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.

You don't!!? Then how do you keep track of fluid intake/output to make sure your kidneys aren't shutting down?

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!

It wouldn't go up noticeably at all. As that level rises, it adds pressure to the other side of the S pipe, making that side overflow into the downpipe and towards the sewer.

You only measure urination? Amateur!!

I use a woodworker’s moisture gauge to measure the water content of my stool.

Ha! Stool-science is a hoax created by big toilet

S pipe

Is that the one invented by a guy with the name Crapper?

If you want to measure your urine for whatever reason, there are a couple other options. One is you could just time your stream a few times along with total volume. Then you can use time as the surrogate for volume. Another more accurate way would be a precise digital scale. You just need to be weighed before and after, assume a specific gravity based on color, and you’ll have a pretty accurate measurement of volume. But why?