Strangely, I've always (and considered for myself) that this point is 94°.
We are here running a single category with 13.5 boosted. I've made a lot of test between motors and clearly that's the first parameter you have to consider. Then comes the ESC, which will allows you to properly control the motor. In this configuration, ESC are not as sollicited as motors. Motors will be the primay heat source, that's why you'll need proper fan on it.
Then you have to consider the track. If you can split the track in a slow technical layout and a fast straight lane or a very fast section, it means that you'll need boost and turbo to reach optimal speed. If you are running a 100% technical track or 100% open track, you can go with the boost only.
Mechanical timing can be adjusted also. Most common values are between 30 and 40 degrees, also based on track. Ratio is very important also, it can vary between 6.7 and 6.2.
At last but not least, you have to consider the punch parameter (punch or power or DDRS, whatever) because if you wheels are slipping, the motor will heat more.