There's not really formula or anything. If you're working on billable matters, bill the time you work. If the billing partner wants to right off your time, that's his business. But, we aren't nuclear power plants, operating at 73.4% efficiency.
It's more a question of 1) how often do you not have the motivation to do work / get distracted and so waste time and 2) how many non-billlable commitments do you have.
On the second point, this will vary wildly by firm, I suspect, but something like 5 hours per week on average might be typical. Department lunches, CLEs, in-office trainings, entering your time, recording your time, responding to non-billable emails, attending summer events, driving to and from work (if you count that as "work")... it can add up. Also, if your firm doesn't count pro-bono as billable but nonetheless encourages you to do it, that can also be a time commitment. At the peak of summer when you have 3 summer events a week + 2 summer lunches a week + a bunch of other stuff, you could easily spend 10-15 hours on non-billable stuff.
On the first point, it partially depends on you, the cases your on, and, I suspect, the face time requirement at the firm. If you get in to work at 8:50, are you the kind of guy (or girl) who strolls in, gets a cup of coffee, reads cnn.com / espn.com / abovethelaw.com for thirty minutes as you wake up? Similarly, do you get coffee every few hours, text your GF, talk with your buddy down the hall, etc? None of that is billable, but you're at work and if you think of that stuff as "working", well, then it could easily waste 1-2 hours per day. The second and third points are interrelated (projects + face time requirement). If you are given a lot of small tasks and your firm has a strong face-time requirement, you'll be stuck at the office even when you aren't working. On the other hand, if you're often given 2-week long projects and the firm has no problem with you leaving at 5:00, having dinner with your wife and kids, then getting back online at 8:00, well, there's no reason you can't be 100% efficient while at home. I'm lucky that way, and I can lock myself in my home-office and do work. If I get tired of doing that, I go back into the main room and watch TV or whatever. No reason I can't work in my home-office while I'm at my home-office. I can't speak from personal experience on this point, but my guess is that if you're at a firm with an implied requirement that you stay at the office from 8:00 - 7:00 every day, you'll get mentally fried by about 4:30 and your efficiency will drop. By that I mean you tell yourself "fuck it" and read espn for an hour, then feel guilty, and get back to work.