Determining actual temperature of the heated bed

I just got started with a Makerfarm 6" Prusa i3. I've gone through calibration but did some checking on the heated bed and found significant differences between the thermistor readings (on the bottom of the heated bed PC and the temperature on top of the glass.

When I set 60C for printing PLA, my actual temperature on top of the glass ends up significantly lower. To get an actual temperature between 58C and 70C I have to set the bed temperature to around 90C in pronterface. I get around 12-15C variation between the middle of the heated bed and the edge where my measuring thermocouple is clamped.

I've checked my readings with a thermocouple on a digital multimeter as well as with an IR thermometer.

I'm just wondering if others have found similar results and is this just another calibration step?

I am actually surprised it's only a 12-15c difference. Mine was more like 40c difference. I added a silicone thermal foam (1/8") between the heated bed and the glass. It's very thermally conductive and it conforms to slight differences in level of the heat bed.

I now have mine only 9c away, the temp is much more stable across the entire glass and the silicone foam acts as a little shock absorber as well. My prints have never been better but it takes a few more minutes to get to temp.

The silicone foam sounds like a good idea. Looks like you can get it from Grainger here in the US in 1/8" thickness. I'll give it a try.

I'm not sure my measurements were precise - I mostly used the IR thermometer for spot checking and didn't bother taping down multiple thermocouples.

Right now my biggest problem is with the hot end and PLA...