I agree with NS1 that the Max reads close to real as any I have seen (even tho I dont know of any that you could call accurate).
The 10~15° lower than actual fidure came from a test done by the bleeding edge site and was the difference they saw compared to thier proble which was paced in the stock HS 1/16" above the IHS. With the reading coming from a probe in the actual HS it was probably repeatable but would always read low.
The article has been taken down but here is a
Post that has most of the results listed. I can easily believe that thier probe to be arleast 15° low from actual.
Here is a shot I took of my MBM% hi/lo readings for the last few days with my 2.4c on my IC7 Max3 with the average ambient in the mid to low 60sf and 69° when the shot was taken. I know when my room temp is in the low to mid 70s (normal summertime temps) my cpu averages temps in the high 40sC and my prescott read about 8° higher.
I hope this info helps.