Playing some more with monitoring the Pi temperature and clock frequency and research. With 'lots' of multiplies I got the temperature to rise ~10° C and the clock to drop from normal (maximum) of 1500MHz, to the minimum of 600MHz in a single step on a few occasions. An excuse to play and analyze.

The hard frequency throttle temperature is 85°C and the default soft throttle temperature is 60°C. To avoid a drop in clock frequency, I think a temperature of 58°C to start the fan and when the temperature drops back down to 45°C turn the fan off sounds right for a start. I'm guessing the fan will seldom come on in the automatic mode. 

It seems that when running their end-of-year programs, Tomar coughed up a duplicate of my order for sample semaphores. This would be really great except they did charge my card again. Anyway a total of 32 semaphores will be needed at some point.