Addendum for achievement: Connect to Tor from a public accessible network/wifi that is free from surveillance using a pristine installation and never use that network-device again.
Addendum 2: If you use the network device twice, you may achieve only pseudonymity.
If you choose to communicate a second time from the same endpoint with the same equipment you may achieve only pseudonymity.
Since Tor doesn't limit the encapsulated protocols, it depends on the implementation and awareness of the user and you can't put a number or percentage on that.
Imho the Tor-role has changed, it provides access against censorship, DPI, region-partioning and hidden services. Simply try to access youtube or any other global service via different tor exits, that may be intresting, not from an anonymity point of view.
At this event your anonymity becomes a pseudonym.
The next step would be to try to reproduce or predict behavior and setup a trigger for that information.
If the loss (compromise) of anonymity or pseudonymity may lead to imprisonment, torture, assassination or death this maybe an issue to consider.
If you try to obfuscate your access to porn, it is a completly different story.
Here you go:
http://diyhpl.us/~bryan/papers2/security/Towards%20an%20info...
http://diyhpl.us/~bryan/papers2/security/Towards%20measuring...
http://en.wikipedia.org/wiki/Degree_of_anonymity
Also this is fun: http://www.gwern.net/Death%20Note%20Anonymity