How did you manage this? I currently work 80%, but I don't see a super-viable pathway to go down to 60%. You work three days a week or what? (Mostly four days a week in my case, with some additional days taken as holiday).
I work 5 hours per day, with 4 hours on Fridays. We did it so I can work on a side-project (writing a book) and so our boy don't have too long days at kindergarten.
We do live in a small village where our cost of living is quite low. I also work remotely and keep a fairly good salary comparatively, so it all works out well.
Yet my output isn't very different from what I used to produce, so your theory seems to match my experience.