I work 7 hours per day Monday to Thursday, and Fridays I alternate between 8 hours and having the day off. Over two weeks, this equals 80%. If needed I do some work on my "free" Fridays as well, but it's usually not needed.
If you ask me, I'd say I'm as productive on this schedule as I would have been on 8 hours per day - if not more. So it's win-win: my employer gets the output of 100% but only pays 80%, and I get time off to do other things.