The setting on any refrigerator can vary. There really is no best setting. It's whatever setting that works the best for you. Due to the different conditions that occur, is why the controls are adjustable. In this model, the freezer control will adjust the air flow (damper) and the refrigerator control, is for the compressor. If both sections seem warm, I suggest turning the refrigerator control to a colder setting but move the freezer control slightly warmer. This adjustment should help both sections cool down slightly more. When making a temperature adjustment, always give it at least 24 hours, before making additional adjustments. The ideal temperature for a freezer is 0-5 degrees and for the refrigerator section is 36-40 degrees. To check the temperature, place a thermometer between some packages (product temperature), air temperature is not a good check.
The length of the defrost cycle in this model should always be the same. This has a standard defrost timer which stays in defrost for 30 minutes. What will vary, is the time it takes to go into a defrost cycle and the length of time the element stays on. There is a defrost bi-metal mounted on the evaporator coil and when it gets warm enough, it will open a circuit to the element shutting it off. This is to protect it from getting too hot. The timer, however, will continue in the defrost cycle for the full 30 minutes. The time it takes to go into a defrost cycle will vary also. This is called an accumulative run timer, which means it advances into a defrost, every 10 hours of compressor run time. If you don't notice a heavy frost build up on the back wall in the freezer, the defrost system should be ok. If you need more assistance, you can resubmit your question with more details.