OKay, there appears to be a bit of confusion and the post is going slightly away from where it was intended.
As far as I understand it:
setting a DH of 1-200' will give all callouts ie 50/40/30 etc etc AND APPROACHING MINIMUMS/ MINIMUMS
setting a DH of 0' will still give you 50/40/30 but no APPROACHING and MINIMUMS, as by definition there is no minima..
Setting any negative value, just removes the bug from the RAD ALT display, and obviously removes the APPROACHING and MINIMUMS call, 50/40/30 still occur though.
My question initially was why have a scale that goes to -20'??? surely an OFF position would make just as much sense...