The delta between the temperature of the NTC and the Tvj differs under different cooling conditions. We cannot give a universal advice that works for all operating conditions of the modules. To accurately determine this delta under certain operation conditions, it is best to measure the temperatures inside a module with an IR camera. To do this you will need black painted modules with no gel inside. Other option is the modules with thermocouples glued to the chips inside the module.
You can order both options through your sales channel.
Some further details about the use of NTC you can find here.