I've been having adhesion problems with filaments that need a specific bed temperature, ABS etc. so I used a couple of means of measuring the actual temperature at the bed surface and it's way cooler than the display indicates. Set 120 degrees and the actual surface temp is about 95. Given the relatively low thermal conductivity of glass I would expect a gradient between the heater and the glass surface but surely that would have been factored in. Do I simply have a bad thermistor? Should I be adding 20 degrees or so to the recommended bed temps? How critical are they anyway?