A day rate means a day's worth of your time, not necessarily of your code, and a standard workday normalizes to 8 hours. Don't sweat time on keys and be as productive as you'd normally be. If you'd normally take periodic breaks, take periodic breaks; the same goes for other work habits. If you can't code continuously for 8 hours (and most people can't), find other ways to contribute your skills. I believe this is fairly standard consulting practice.