for the most acurate results thermal tape it right under the center of the cpu. this is tricky as you have to carefully route the wires
or you can thermal or electric tape it as close to the chip as you can without covering any of the resisters or whatever they are on the top of the die the latter will give you temp readings about 2-5 deg c low and the first will give you 1-2 degree c lower than actual
have fun