You can reset the zero position on those with the (aparently missing) tool. It slides into the small slot on the top of the caliper, behind the dial. You open them until the needle is whee you want it, press the 'tool' into the slot (disengaging the pinion from the rack) and close them down. Then remove the tool, and it should be fine.
I have the same ones, but slightly newer (grey box not the red box).
Worth 50 all day long! Someone grab these!