The reliable way would be to place a thermalprobe under the heatspreader of the cpu. I think heatsink reviewers out there do that.
I can try to do a idle calibration when I get home with realtemp. But the would only shift my temps so they are more correct at idle. I need to add probably 10 deg C on core one and 5 deg C on core two.
That takes care of idle temps. But that just puts my load temps more 30 degrees above water temps. Which would indicate my waterblock is the worst in history (it might be), but I seriously doubt that. I will measure the temperature of waterblocks copperplate. I bet its very close to watertemperature.
And if it turns out as I expect that leaves me with the conclusion that the integrated heatspreader(30C temp delta, eeeek) is crap or internal temperature measurements are useless.