From Preferences & Costs, select "Preferences," then "Single Part Material Weight."
This value is only used with single part nesting and defines the way that single part nesting calculates the nested weight of a part.
Sheet Weight (Preferred)
The Nested Weight of a part is calculated by determining the total weight of all the sheets requited, and the dividing it by the number of parts required.
The sheets will be cropped using the horizontal and vertical cropping rules. see Nesting
Parts Per Sheet
The Parts Per Sheet option calculates the total weight for a single sheet and divides the weight of the sheet by the number of parts that fit on the sheet.