The way I look at this- The leadscrew on my lathe (and most) is a single start thread. EVERY time you drop the half nuts, they're in the same thread. You're not looking for a particular thread. That gear and dial are doing math (mechanically, like an old adding machine), to tell you when you've been the correct amount of revolutions to to have the chuck and the leadscrew come back to the same exact relationship as when you first started. Or some acceptable fraction thereof, depending on the thread pitch.
If you add the 127 tooth gear, you have fundamentally changed the geared relationship(s) that the threading dial depends on, by adding a large, prime number into the mix. A prime number that is very incomatible with the mechanical counter, who's mechanical "software" still thinks it's counting four inches per revolution, but now you're "inputting" millimeters.... You'd have to have a minimum 127 leadscrew revolutions, or 127 teeth on your thread dial gear, to get one single line on your dial, to achieve -any- useful realignment of the chuck and the leadscrew, and I think (I'd have to work on it, it's not lazy math.....), I think it would probably need a geometric multiple of 127 teeth. And still have to wait for your exact number to come around every time. Even at 127, that's gonna be a big honkin' thread dial.
