I had time recently to look at the Interrupt routines I want to use; got a couple of ATTiny examples working, one for timer and the other for a pin interrupt.

I will use sleep mode for the times between readings, but one of the relatively hight power modes that leave the PWM running. I might investigate using super low power mode for battery low conditions, in these modes even the PWM is off, but if the fan is running I need to limit sleep mode to one of the mid states.