Selecting records based on a sub total

I need to create a listing of sales reps and their customers who have spent over $300 in a selected date range. I am grouping by the sales rep id and then the customer id. However, I'm not sure how to only select a customer for each sales rep who has spent a total of $300 or more in the selected date range. If I put in the record selection formulas the formula, AmtSpent > 299.99, then I miss getting customers who have several orders for under $300, but together they add up to over $300.

Any help would be greatly appreciated.

-Rick Hardy