Thanks Jim for posting that video! I can't believe how accurate it is.
A digital micrometer may make it easier to build a Hacklinger gauge. You can get one for $34.95 at Harbor Freight. Cut the C section off and attach it to a non ferrous rod which attaches to the extractor magnet. You wouldn't need to deal with the screw adjustment anymore since it is already built into the micrometer. Just have build a frame to align the extractor magnet rod to a compartmentalized clicker magnet.
Edit: This would work only if it has a "zero" function, which I am not sure if it does. Anyone know what the "set" function does?