I feed my corns based on weight.
At 300-450 grams for the snake, I give a 25 gram feeder every 14 days.
At 500+ grams, I give a 25-30gram prey item every 21 day.
I feed mice or rats, whichever the snake prefers, but I don't switch back and forth as that can sometimes cause refusals.
You want to keep your Corn shaped like a loaf of bread: rounded top, straight sides, flat bottom. Too angular, feed more; too round, feed less.