Dear Sir, its been now 5 years i'm working in OpenERP full time, started from OpenERP's development center , i worked there on OpenERP client projects, and then worked for OpenERP partners, it was version 4.2 then after worked in 5,6.0, 6.1, 7.0 and recent release 8.0, etc, and i have worked on projects of customization of Accounting module, and HR payroll etc module also ..... i checked the document, and read it 2-3 times, sounds little complex but interesting, there are some questions like where you want field to specify grace days , ( it can be in payment term it self if its so general, or can be in customer record so can configure for each customer separate ) , and for performance issue, i'm thinking to calculate interest on fly, so it wont update occupy resources to update 100 of invoices every day, instead whenever that field accessed, it'll calculate on fly, one big question, for which OpenERP version this has to be done ?...... Zahir