I have had good results with INS_TCALn_TMAX = 60 [°C] so far, even with boards without a heater. If it doesn’t reach the 60°C it just stops after 10 minutes without significant temperature increase (< 0.5°C).
Regardless of that I would recommend to follow the Temp-Cal section in “How to methodically tune (almost) any multicopter using ArduCopter 4.4.x”, I know you know the guide.
It works for a complete drone exactly the same way as it would for a single FC (except you need more freezer space) and by using the offline calibration tool you are able to verify the quality of the calibration.