Inch and a half of substrate is one problem.
Temping the surface of the substrate is a second problem.
Feeling with your warm blooded hand is also yet another problem.
Setting up heat for these animals is not that hard, I still cant fathom why everyone overcomplicates it.
You NEED to monitor the tempurature of the hottest surface your animal can touch, this would be when they burrow OR under the surface of the substrate.