Instead of using all the formulas, I just bought a 1/2 watt pot, built the thing, and adjusted it and tried it until it cut off at my required voltage. Then I measured the resistance of the potentiometer and bought a resistor to match it. I then gave all the parts including the resistor to my son and he built him self another one. My son is 11. Different relays have different coils so this seemed the simplest way for me. It has to be a low resistance pot, like 500 Ohms or lower if you can find one. I think I ended up with like 150 ohms for 5.0 volt cut off.

I also used a digital meter with a min-max function so I could see exactly where the relay opened up. Made it easier than watching it.

By the way they both work flawlessly.

David Root