Doesn't the P4 1.6 have a sensor inside? This would be your best measure. Second option would be a thermal probe well located. Last option (in terms of accuracy) is Motherboard Monitor (software), it reads the thermal probe of the mobo. Also temps recorded from the first to the last method are decresing, because you are not so close in each one.
Well, for the cm2 you take a ruler ;-)
The rest I think it's mathematics.
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