During periods of cold chickens as well as any other war blooded animal kept outside will increase their caloric intake which we see as a higher feed bill during this time. Calorie for calorie fats and carbohydrates provide a more efficient fuel than protein for this purpose. This 10% rule imo can be chunked during the winter. Unless more corn and oats are thrown than can be consumed in about 15 minutes, even 2 or 3 times a day during the middle of winter the flock will still visit the feeder enough inbetween to get adequate protein to maintain muscle.
Here is an interesting
link comparing corn and oats. If you want to keep the chickens warm throw oats.