you also need to consider how you are heating the viv, as is if its a heatmat underneath the tank, or in the tank but under the substrate, then excess material can cause the heat to become trapped. This could lead to overheating if a thermostat wasn't used.
I use about a 3/4 inch thickness of Aspen.. too much and you can insulate the heat from getting through from the mat. Too little and the snake can get burnt - especially if the mat is in contact with the tub bottom.
I suppose different people have their own methods, there probably isn't a right answer.. If you ensure that you have a good digi thermometer and place the probe on the surface of the substrate where the snake will lay, then you can make sure the temps are correct for the depth of substrate you choose to use.