Calculate days between subtotaled list

[hutch@edge.net]

Good day, Gurus.

Not sure if this is best accomplished via VBA or Formula, but I know all the smartest people hang out in the Programming forum, so here goes.

In the attached workbook, I have subtotaled a list of parts. I want to add a formula beside the subtotals, under Column C, "Days BO", that subtracts earliest date in column R from the latest date in column R, for each part number.

I have added a simple formula to the attached example that does what I want, but the full sheet will be thousands of lines weekly, with varying part numbers, and different counts. I don't know how to take the count function into account so I can copy a single formula all the way down the sheet.

Thanks in advance for any help you can offer.