Hi Paul,
No such report is available. You have to go for development. KIndly contact your ABAP team and explain the logic when GR amount in LC is 80% of PO gross amount then system should send mail i.e alarm to the buyer/user id to be maintained in some table. Along with mail a report should be attached likewise.
You can use some badi ME_PROCESS_PO_CUST.
Regards,
Priti