Part deductions (also other things like waste yield) can happen in different places, but most of you will have it set up so that your deductions happen at the attribute level. To change these types of adjustments, you'll go to Settings --> Product Attributes. Then click on the attribute you want to look at. Scroll down to Selector Settings and you should be able to see a table of information. This is where you will be able to edit your deductions.
Here's a short video to help guide you to this point: