As with any balance of the installation site is extremely important. The scales that I used as stand on tables with steel frame of a concrete slab and as a conclusion a marble slab. If you compare the balance now on a wooden floor, this is more or less strong. It is better then a concrete floor with tiles.
The load on the weighing surface plays an enormous role in the measurement. If you want to perform good reproducible measurement, one should be aware of this. So your feet always in the same place!
Therefore, you get a good read, the above conditions for a good balance for 30.