What I do is remove all the bark substrate from inside my hide and just have some newspaper there. That way they are getting the heat that the thermometer probe is reading. Theres no real need to have the bark inside the hide, the bark is mostly just for show and holding humidity, both of which are an affected by removing the bark from inside the hide.
With this solution your snake has the tamp that it needs inside its hides and you don't have to worry about how much substrat to put in there over the heater.