It must have been close to useless Ed if you weren't entering the dia as a negative.
I have the carriage travel on mine set to read larger as I move toward the spindle. Face a part, zero the scale then I can cut to give a shoulder at any distance or bore depth in positive numbers.
BUT if your facing a part to a desired thickness, say .500, take a pass and measure the thickness as say .555, you need to enter it as a negative so that the numbers get smaller toward .5 as you remove material.

Setting up the Fagor on the Summit is less forgiving. I have a scale on the carriage and the compound. The readout will display either one or the sum of them. The carriage is too heavy to get to a thou easily so I keep the compound parallel to the bed, get close to a shoulder with the carriage then fine tune with the compound. But you have to be sure they're both reading in the same direction or one will subtract from the other when moved in the same direction.