expense : product cost precision

Hi, We are just starting and testing OpenERP on our server and it looks great. I'm dealing with expense report and product price & cost decimal precision issue: In Configuration => Technical => Database structure => Decimal precision, I have set decimal precision of product price at 3. I have created a product for Car Travel and in the costs i have set 0.536 When I start a new "expense report" in the HR module, when I select Car Travel, the Unit Price is autofilled with 0.540 instead of 0.536 ! How to fix it this ? Thanks for your support.