I do that same 20 minute routine. I use an app on my phone to keep track of time spent on projects/automate the timing.[0] Without it, I get so little work done.
That way you get not only the benefit of the alarm at the end, but also a visually distinct display of how much time is left if you ever look over at the timer while it's counting down.
May be more useful to me than most because of ADHD and the time blindness that comes with it, but either way, it's been a great help and a way to avoid paying attention to my phone while working.