By the way, it is common to be paid far less for 40 hours than you should receive as well, and plenty of people work unpaid overtime who would be happy just to work the 40 hours they get paid for.

Salary negotiations are always tricky, especially if you don't know your true market value. Given the difference between compensation at different companies one way you could attack this problem is to find a company that will match your current salary but at a reduced number of hours. Employees have the same problem that companies have: it is hard to charge by value if you don't know your value.

As a gardener my work is worth less than as a computer programmer than as a financial advisor. Because each of those domains values time entirely differently, and because there are different levels of competition.

If you can show your boss that if you work less that your productivity will likely remain the same or that it will go up they will probably wonder why they've been paying you for all that extra time in the past... so instead I would just argue to try moving to a different company at a better salary for the same amount of work or for the same salary with less work.