Ideally, the harness should be taut, with no slack. We recommend you read the following article: ‘These are the reasons why a tight harness is a safe harness.’
If there is any slack (even if only a few millimeters), the occupant’s body will move this distance before the belt then holds the occupant in place in the event of braking or a collision. The result is that the harness itself will cause injury, and it will also work incorrectly. We know that a harness is tight enough when we can only slide a couple of fingers under it.