With a good heat sink and fan, the heat sink should be warm to the touch..
Without the fan running, it should get pretty hot.. You should check this to make sure there is good contact between the CPU case and the heat sink.. Turn the fan on.. it should cool down to where it barely feels warm....
This is the best gauge you can use...
I feel you are waisting your time and energy trying to be more precise than this..
CPU temp is rarely what is standing in the way of successful overclocking..
Though people who want to sell you cooling solutions would have you
If you are going to start spending a lot of money on temperature probes,
and cooling devices, why not just by a faster CPU?
Like any electrical device, the cooler it runs the longer it will last..
So if you lower the temp 10 degrees F.. it might last 20 years
instead of 15... Are you going to keep your CPU for 15 years?
That would be the only reason to try to keep it cooler than what
you get with a simple heatsink and fan...