However, to change zero-set reading, like for weighing milk (in a container), you would not be interested in measuring the container weight and then deducting it. So, put the container on the scale and press S1. Container weight will be added to zero_set reading and a new zero reading will be shown. Now, pour milk in the container and read the weight. To remove the container weight from zero_set reading, remove the container and press S1 again.
Change this factor suitably by putting a known weight on the load tray the first time. Once this is done, your machine will work fine. This is explained in the source code very clearly.
Fitting of the scale
If you look at the load cell, it has strain gauge attached on all three sides with suitable strain-relieving frames inbuilt in the design. Attach a suitable tray on the load end of the load cell for holding the object for weight measurement. Put a thick washer between the base plate of the tray and the load cell, and tighten firmly. Your weighing machine is ready for use.