Hi Jake,
Here's what you need to do:
1) Create a formula called 'FirstDayOfWeek' with this code:
{Date} - DayOfWeek({Date}, crSunday) + 1
This assumes that the week starts from Sunday.
2) Create another formula and place this on the GH1:
Count({PartID}) / DistinctCount({@FirstDayOfWeek})
-Abhilash