The height of the drop must be related to the thickness of the glass.
The glass moving at the edge of the hole becomes thin, so the deeper the drop, the thicker the
The general rule of thumb is to have 6mm for the first 50mm drop. For each additional 50mm an additional 3mm of glass is required. So a 200 mm drop will require glass at least 15mm thick. Observation and experimentation is required to determine how thick the glass needs to be in relation to the length of the drop, but this is a place to start.
The thinning effect of the stretching can be influenced by both the temperature and material of the supporting material.