I use a similar technique, combined with meticulous time tracking of every and any minute I spend professionally. As somebody who is self-employed, I quickly realized that working 8+ hours per day is an absolute myth. I count any day where I spent more than 4 hours in a state of focused work as a great day, and weeks where I spend more than 20 hours a week are rare. My personal goal has shifted from trying to work as many hours as possible to compressing these 4 productive hours into the smallest time window possible. Arriving at 10am at work, and leaving at 5pm is close to the optimum in my experience, due to coffee breaks, lunch, and the occasional goofing off on HN.

So no, you're not cheating the system, you are probably running at optimum efficiency. The efficiency of the average employee in terms of productive hours per day spent in chair is ridiculously low.

As a side note: I'm very much wondering whether I should insist on 40 hour work weeks when I get to the point of hiring people. I know there is a movement going for the 4-day workweek, but I personally see more in a 6 hour work day.