No formula? Measure it again and again instead

1.7a combined uncertainties from a formula. But a screw gauge measuring a wire's diameter has no formula at all — just six slightly different readings of the same wire. Where does the uncertainty come from now?

The answer is simple: measure it several times, average the readings, then look at how far each reading actually strayed from that average. That spread itself becomes the uncertainty.

Timing experiments use their own version of the same idea. Instead of repeating one instant, you count many vibrations at once, and the stopwatch's least count gets divided across all of them.