Create a new summary field which summarizes the part field as a running total; then create a CF like (say) “myNewSummaryField = 10” (or 11, depending on your formatting method). To CF all parts within the TOP x, create a CF based on “myNewSummaryField <= x”.
Sorry, made a mistake; I just realized that of course the part name (or ID) will be in all the summarized records, so that's no help. What I did was to create a calc field Case ( PartID ≠ GetNthRecord ( PartID ; GetNthRecord - 1 ) ; 1 ).
This lets you create a result which is unique to each new start of a sub-summary, and which you can summarize. Obviously, the field to check is the break field for the summary. If anyone has a better method, please let me know.
btw, for a simple horizontal ruler, add a button element (assign it no action), adjust the dimensions in the Inspector (like height: 2pt), and CF it. FileMaker doesn't offer CF for the simple graphic elements from the Toolbar, but it does so for buttons, which you can then mis-use to your heart's content