Estimates, orders, invoices

In a distribution business we have estimates, orders, and invoices.  They often progress in that order but often just go from order to invoice.  What is the most efficient way to track open orders to facilitate sales and cash flow forecasts?
 

Currently I am entering all orders as estimates.  This gets confusing as the active estimate column does not always clear after invoicing has taken place.  Also a listing of open orders would help to ensure drop shipments are invoiced properly.