Here is a good way of marking the water level in a flood test.
But I'm happy you finally omitted that 'Most Places' (Canada & USA) require these flood tests. It most be that silly code that says all shower pans need to be built so they hold water. I might be just fishing here - but I think a flood test proves this. Imagine that - filling a shower pan to see if it in fact can do the job it's required to do.
You know what would be fun? If someone reading this back and forth bander (thinking we are a couple of school kids debating) just tries there own experiment and goes over to the John Bridge Forum and starts a new discussion called "Should I flood test my shower project?". Then sit back and watch how the posters have a field day. How many will say "Yes - It is required by code" and how many will say "I use Kerdi every month and if you use a sheet membrane it is kind of overkill". I know where my money is.
Flood test are not useless Jim - they are a code required step in shower construction. And of course you would protect them after doing them, although that is not written into the code books but just good old common sense.