Just wanted to add something I left out of my first post for anyone else that is using this.
[
play.google.com] Screen Filter
I use this app as well with the timer. Before I go in, I turn on Screen Filter to a very low setting. The screen is nearly invisible at that point. Then I just tap the screen in the same spot to add a marker time each time someone comes to the table, or shortly after. Works from the seat of a booth or on my lap quite nicely, and as with all things smartphone anymore, no one seems to think twice about someone fiddling with a phone.