Average Days

I am looking to have some metrics set up on a dashboard all around average days to close. I have a column for a sales person, two date columns (one a start date and one a close date) and a column called Workflow Status. How can I do the two following things via formulas:

1. Filter this first by the workflow status being equal to one specific status
2. Show the average close date by year?
3. Show the average close date by sales person?
4. Maybe even break it down by both year and sales person?

I was thinking of using the Netdays but I am unsure how I can filter that by the critera I am looking at.

Thanks for the help.