I look at it like this: I have days where I accomplish a ton (enough to surprise myself), and I have other days where I do very little. This field requires so much creativity (every day is different, we aren't just assembling the same widgets day in and out), that it's just not possible to be 100% 8 hrs a day. So, I'd suggest evaluating your performance on a weekly basis. If at the end of a week you can look back and see you carried the ball farther than it was at the start of the week and you accomplished at least a few things on your long term goal list ... you're doing fine/average. I hate people that are too uber productive, they fuck it up for the rest of us. ;)