unfortunately, there's a tons of "unknowns' and there is no known offset. Everything from ambient case temp, airflow, what your current readings are, the cotnact with the CPU back, whether or not you put a bit of grease on the thermistor tip, and even heatsink airflow mess with this "offset".... It could potentially be 3-12C off, depending on the above conditions, or possibly more 🙁
Mike