What Search Criteria to add to only show Estimates that have no associated Sales Order yet?

We are attempting to automatically create sales orders from estimates. We have a workflow setup that will successfully transform the estimate into a sales order based upon a saved search.

The problem is, the saved search will continue to show the estimate after the workflow has completed and created the sales order. If the workflow was released as opposed to being in the testing state, the estimate would get multiple sales orders created because it would always show in the saved search the workflow uses as criteria to run.

We have tried every Workflow History criteria we can think of to get estimates to disappear from the saved search after the workflow has completed. None of them have work. The estimate always stays in the saved search, so it creates a new sales order every time the workflow runs on the schedule.