I knew there was a formula, I kept getting stuff about air conditioners and science projects when I searched.

Yeah, It reads a coolant temp. sensor and controls the radiator fan speed. This is my first time using PWM, got started yesterday on a breadboard with a 80mm PC fan. 4 kHz was noisy, 20kHz is working real well. I’m more excited about PWM than I should be, I’ve been connecting other things to the circuit to see how they respond.

Ordered some MOSFETs , looks like they will be useful for all sorts of mischief.
Thanks for you help.