OOD Code for Specified Races

We use OOD (Average of all non discarded races). Works ok, but we would rather get an average of all races excluding DNC and other codes such as OCS, DNF,RET etc. This is because we have a generous discard profile for club racing and the average only takes in to account non discarded which biases up the average. One way round it is is to only use DNC for a RET or DNS etc but wondered if there was a way to do it. Not sure what ‘Average of Specified Races’ is as cant see where to specify? In a perfect world it would be an average of non coded results?