Average order value for new customers

We are working on some metrics. We are looking to get the average order value over a time period for new and existing customers. We are struggling with what to use in the saved search to indicate we only want new or existing customers.

What criteria would we use in order to have one saved search that shows AOV for new customers over the time period and returning customers over the same time period on a different saved search? We have not been able to determine what field would work to filter on.

We realize there is a canned report for new customers that can be used. That requires some manual work. It has to be exported to csv so the average order value can be calculated. We could then take our total sales over the period and subtract the amount from this new customer report, but that will not allow us to get the average order value, as we would have no idea how many orders were placed by existing customers.