Thought I'd throw some numbers I recently recorded into this thread: I've been using a 71C TRD Thermostat for ~55,000 km in my Ron Davis Rad.
Recorded using TOYOBD1 Datalogger: Cruise coolant temp appears to hover aroung 75-78C. Once stopped at idle, it climbs to 94.8C over the course of 2 minutes, at which point the radiator fan turned on. Within ~10 seconds, the rad fan would turn off and the temperature would read 92C. This would repeat as the temp climbed back up over 94.8.
These temperatures are measured in the coolant neck (correct me if I'm wrong).
Inside the car, the stock temp gauge sits left of centre while cruising (so ~78C I guess), but quickly moves to the middle (presumably around 95C) when stopped.
This was taken 3 years ago while driving through the mountains in the middle of summer. I took it because it was the coolest I had seen the temp gauge dip and it was during a long downhill portion of the drive at idle. It normally sits slightly higher than this.
I just read the BGB coolant system description. The rad fan is designed to activate at >93C coolant temp (measured at the radiator temperature swtich), and it is supposed to stop at <83C.
I'm thinking about splitting the difference and trying the ATS 76C thermostat next.