In Stock number in Sales Order Screen includes non-nettable warehouses

The "In Stock" column displayed on the Sales Order row seems to show the total stock for that item across all warehouses.

Is there any way to restrict this to one or more specific warehouses (e.g. to exclude a 'Scrap' warehouse). I assumed that "In Stock" only counted stock in Nettable warehouses, but this does not seem to be the case.

Any help appreciated.

Chris Burr