heated bed is not so heated

I have 3mm aluminium plate which has silicone heater from Alirubber. The bed heats nicely and thermistor measures right temperature. If I want my DMM temperature probe show me 70C I put it under the bed when bed is heated to 70C. On top of the bed I can measure only 55-60. I'm not that interested the heater's temperature as long as it stays less than 120C. What would be best way to get this problem under control?
Another thermistor to plate? Writing some calibration chart and heat the system to higher temperature?

Um, if there's a 5C offset, set the bed temperature 5C higher.

My bed heater reads 95C when the print surface is actually at 105C. It doesn't bother me enough to use a different thermistor or try to edit the thermistor table. I just set the printer to 95 when I want the bed to be 105C for printing ABS.

There are so many different thermistors in use that you really can't trust the temperature readings from the controller. I think that a lot of the companies who make heaters with integral thermistors use whichever thermistors they can buy cheaply. You have to experiment a bit to see what temperature settings work.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]