Tempered glass can be tricky, because some types admit less IR light than regular glass. The “active” sensing part of the sensing tends to be diminished, since the IR light that is emitted has to make two round trips through the glass.
What we would usually recommend — if at all possible — is to add some supplemental light sources outside of the glass, to allow it to be stimulated by passive sensing as well. Typically, that would be something like a halogen lamp or near-infrared illuminator (the kind used by security cameras; usually LED-based and invisible), pointing at the table top.
For a programming approach, yes adjust the cutoff values.
