I would assume the max core temperature is the TjMax used to calculate your core temperature. That is the point where the temperature delta value read from the core sensors reaches 0 and the CPU starts throttling. So a safe max core temperature would be around TjMax-20 °C or TjMax-30°C.
If you use the Open Hardware Monitor
to read your core temperatures you can find the TjMax parameter used by double clicking on the core temperature sensors (or right click -> Parameters...).