Interesting technique. I usually just put a small chunk of 2x4 next to my target piece and use set focus on that.
The bigger issue here is your laser focus. You’re outside the effective focus depth of your Glowforge (by about 0.8”, or in this case with your paper method maybe 1.3” unless you set the focus height manually in your settings) so you’ll end up with a soft engrave. If you want to focus on the bottom of a bowl you need to get creative: