We have finished our Sprint in half the expected time and now we want to compute our focus factor to use in the next Sprint. Though, by doing the math by the book, we get an awful number. How are we calculating it wrong? Please take a look bellow:Team composition: 3 Sprint length: 2    weeksTeam availability: 28 days(discounting team members shared withother teams)Estimated focus factor: 70%Estimated velocity: 19 ideal days(derived from:  28 days * 0.7 focusfactor)Actual velocity: 9.8 days (wefinished the whole Sprint in half theexpected time, clearly overestimated)which gives us a focus factor of...Actual focus factor (to be used inthe next Sprint): 35% (9.8 actualvelocity / 28 available man-days)Acording to Scrum And Xp From The Trenches we calculate Focus Factor as (actual velocity) / (available man-days). Though, by finishing the Sprint in half time and doing the math, we get a focus factor of 35%! What are we doing wrong? 
Calculating last Sprint's Focus Factor
agile;scrum;sprint
The only thing you're doing wrong is measuring velocity in days.  Velocity is measured in points per sprint.If your velocity (based on several completed sprints) is, say, 10 points per sprint, you can pull stories that add up to 10 points into the following sprint and that's it.The problem is, what do you do when you don't have such statistical data on your velocity.  Kniberg deals with it in his book by suggesting that you estimate in man-days  initially, but switch to points per sprint later.The focus factor is simply the percentage of clock time that you can devote to active work on your stories during the sprint.  70% is a good approximation.ADDED to answer comments: Henrik Kniberg in his Scrum and XP from the trenches book suggests that if you don't have velocity (points per sprint) statistics yet and don't have a good grasp of relative story sizing yet, you can approximate it by equating one story point with one man-day.  Because you won't be able, realistically, to devote 100% time to working on stories (meetings, interruptions, etc.), that's where the focus factor comes in.Focus factor and the story-point-person-day equivalence go together.  If you (don't) need one, you (don't) need the other.  If the team is ready to estimate relative sizes in points, they don't need focus factor.