If it's a Chinese mill, it's probably metric. If you're gonna bank on this, you probably want to match it up to a thread gauge. A bolt or a screw with a known thread pitch would suffice in this situation, albeit a little awkward. If the nut is deep enough (has enough threads) for a positive ID, that'd be valid too. And on a
First, (going with 18tpi) turn threads per inch into inches per thread, which is 1/18, or 0.05588. Make that into thousandths, it's 55.555 thousandths of an inch per thread.
Use the spanner notches in that nut to divide 4 sections. (I think there's four, right?...) Then each "quarter" would be 13.75 thousandths of an inch advance per quarter turn. That's a bad number, so I'll insert a fudge factor here where it'll divide out to "who cares", and call it 0.056 thousanths per turn (half a thou liberty there), and it's 14 thousandths per quarter. When you're in the range where you're "close", mark a spot (sharpy is fine, no scribes needed), use a ruler or estimation to mark out those 14 divisions on one of the spanner slots. If they "look" equal- They're equal. Each of your marked divisions will represent 0.001 inches (one thousandth of an inch) of travel. That is the resolution you want to make the smallest meaningful adjustment, without going to the point of getting no returns. Two marks is too big to be considered one "step" of adjustment, and half a mark is pushing the resolution of your adjustments finer than you're going to see any results from.
If it turns out to be a metric 1.5mm thread (very close to 18tpi), the math is exactly the same except it's already in "distance per thread", and it works out (again with slight rounding, done early so it divides away nicely), that's 0.060 per turn, or 15 divisions on each quarter of the nut. to get the 0.001 target resolution. Or any other number you "science" if it turns out to not be one of those two.
If you've got it together and it's good- I'm not suggesting that it can't be. But now or in the future, that's a good way to calibrate your "click elbow" for doing them freehand.