F.28 will not help you, because if some orders with credit issue were released by the credit department, by running F.28 you will block them again if the problem with the credit still remains.
There is already a sample VOFM routine in OVA8 - you can check it by pressing F4 in No credit block field.
Mine looks almost like the sample routine, with just one correction:
My strong recommendation is to contact your local development team and explain what exactly you need to check for credit and in which cases. This is very company- and process-specific and because of that SAP provides only a sample routine and not a configuration option, because you cannot predict all possible business cases.