My company although not implementing a 80/20 rule as such encourages us to consistently keep up-to date with the latest technology, read other blogs and posts such as programmers etc and basically make sure we are keeping our personal development and interests up-to date.
To achieve this they didn't specify an exact amount of time, but made sure that we work on a project and task deadline rather than a by the hour workload. This has meant that we consistently talk with our immediate manager to ensure the tasks assigned to us or we take on are manageable and allow us some time outside of the normal to take for ourselves and refresh our mind and interests. If we are struggling we talk about it to find out why i.e. skill set limitation, too much work, unrealistic deadlines etc
We do keep a rough track of our times spent for billing purposes but this is to the hour on an entire day basis so we are not expected to record what we have done during every minute of the day.
Each of us in the team manages their time themselves so it's up-to them to determine how they do this. Some members of the team work hard at the begging of the week to get their tasks done and any time left over they relax into their own interests. Others like me tend to mix this in during the week as I quite often hit road blocks in my project / task. So at this point I jump over to something else which is quite often my own interest. However as we are task/project driven I still have to make sure I get the job done so it is my responsibility to make sure I don't use all my time on personal tasks at the detriment of the team.
This has in the past lead to some team members rushing their tasks and producing undesirable results. To help with this we introduced periodic peer reviews and also encourage everyone to read other developers code check ins. We encourage an open discussion forum where everyone is free to voice their opinion albeit in a respectful manner.
In the end it came down to a bit of trust from the powers that be, taking ownership by the grunts to ensure the work is done and a good manager in the middle to keep the ship running.