Same here, 8 billable hours (four days a week), 6-8 productive hours, depending on how stressful the day is.
As a data point, I am currently full-time employed, it just happens that I prefer full-time being four days a week. I recently made the switch from five to four, and I'm enjoying it immensely.
"I want to work 4 days per week".
"OK, no problem, we will reduce your salary / bonus etc by 20%".
Time passes.
"Ok, I am doing the same job for 80% of the salary and I never make my bonus".
":)".
I know that being a salaried employee is different, but if you're letting people work you into 10-hour days, don't.
I'm totally with you to prefer a shorter full-time schedule.