Each bag is designed with the ideal baffle height and differential cut to accommodate for the amount of down it needs. Adding a little overfill will get you a few more degrees of warmth, typically 1.5 – 2.5°F per ounce. However, at a certain point there is no real benefit to continuing to add overfill. If you are considering adding more than 2-4 oz. of overfill, depending on the model, we suggest ordering the next warmest bag.