Create a New Commodity called Shrink, as per example below.
In the Input Commodity section, enter the Commodity, Storage and Number of Tonnes you wish to Shrink.
In the Output Commodity Section, enter the same Commodity and Storage and enter the total Shrunk Tonnes.
In the second line, select the Shrink Commodity, Shrink Storage and the amount of Shrink.
Make sure the Total Value of the Input Section is applied to the Commodity in the Output Section.
Click on Finalise to process.