Bo'sun
If you zero at the first edge, find the second edge the display should read the distance between the edges (plus the centre finder probe diameter).
If you them press the 1/2 function key the display reading should halve.
Winding back to zero should place the quill above the centre position.
Questions :-
1) Does the display reading halve when you press them 1/2 key?
2) Does the display count down to zero when you move back towards the middle or freeze on the 1/2 value?
If the reading doesn't halve when you press the 1/2 key odds are the key isn't working properly. Contact issue under the membrane.
If If the reading freezes and doesn't count down you may have entered a lock mode.
My Sino set up has a lock mode if I read the book correctly. Something I make sure to stay the hell away from as the potential for confusion is obvious. That said I can see various uses for it if you exploit the other memories. Transfer of dimensions to replicate feature in other places for example.
Clive