Ability to assign percentages to tags to better reflect time spent

It would be really useful for me if I were able to assign multiple tags with different percentages or a maximum time. For example I use facebook to communicate with a volunteer group, I know that on average that takes up a certain percentage of my time on the site, alternately I could put in a maximum number of minutes I spend on that per day or, even better, per week. So I would have the ability to tag facebook as "procrastination (65%), personal (25%), communication/volunteer (10%)" and this would more accurately reflect my time spent in different ways.