Ideally, there should be enough clearance between the back of the kitchen door and the front of the cabinet to allow the door to open and close smoothly without any interference. The amount of this clearance can vary based on the type of hinges or hardware used for the door, as well as personal preference and any design constraints.
For most kitchen cabinets, a gap of around 2-3 millimeters (approximately 1/16 to 1/8 inch) is sufficient. This small space ensures that the door doesn't scrape against the cabinet and allows for a smooth, hassle-free operation.
In your specific case, with the cabinet frames being 500 mm deep and the drawers measuring 400 mm, it does seem like the slides used (50 mm standard size) are causing a conflict. If you had used 450 mm slides instead, it would have provided a better fit and allowed for a more appropriate gap behind the kitchen door.
As it stands, you may need to consider some adjustments to ensure the door and drawers work together harmoniously. This could involve altering the drawer slides, door hardware, or even making modifications to the cabinet frames if necessary. It's advisable to consult with a professional carpenter or kitchen installer to assess the situation and find the best solution for achieving the desired functionality and aesthetics in your kitchen.