I've got a very similar Minoura stand to that one and it isn't the most sturdy stand in the world. Hard to get repeatable measurements when removing and replacing the wheel. What does help is to make sure the wheel axle is firmly resting in the stand dropouts and the arms are firmly pressed against the axle nuts. You can use the quick release skewer to hold the arms against the nuts but clamp it so that it just rests against the plastic dropouts. Clamping too hard just bends and distorts the dropouts causing the wheel to skew to one side. Practice flipping the wheel around in the stand and see if you can find a method to get repeatable results for dish. The wheel centering device is pretty weak as you have found so either just be satisfied with eyeballing the dish when the wheel on the bike or get a proper dishing tool.