I never give a day rate because I like to come and go as I please. Don't get me wrong i put in a good shift but if I want to finish at 3pm then I can as it's costing the customer no more and no less. On a day rate you would be obligated to work a full day or the customer wouldn't be to happy. .
I'm exactly the same Andy, you don't have to justify your time keeping to anyone. Having said that I generally do more hours on site than any other trade anyway!