So for example if you are firstname.lastname@example.org, and you want your personal calendar named "John Personal" in the Web client - and your web client URL address is webmail.wherever.com
https://[Your Server name here]/service/home/[Your user name or email address here]/[Your Calendar name here].html
you would look for
And - obviously don't forget to url encode it - so the spaces don't cause you a problem.
Lastly - BUT MOST IMPORTANTLY - The user must share their calendar for public viewing. In the web Client - Right-click on the Calendar, and select "Public (view only, no password required)" and "Viewer". Without this step - you will still get the 404 error.
I hope this helps.