To add to the previous replies - see the note at the bottom of this article:
"If you have a grate type that is not included in the available pull-down options, you can either choose one that is close to one of the available options or do your own "sensitivity analysis" which means making adjustments to the structure dimensions in order to get an opening area which is accurate for your requirements."
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.