Toby I assume you are talking about A320.
If I can remember it correctly if you enter a value in the DH fild of the APP page this value is referenced to the radio altimeter and if you put it in the MDA field it is referenced to the barometric altitude readout.
As you want to fly a standard ILS CAT I approach the DA is a barometric altitude and to get the correct indications and voice callouts setting the DA in the MDA field would be just right.
Above might be wrong, as it is long ago that I flew an Airbus...