Calculating last Sprint's Focus Factor

Question

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 weeks
• Team availability: 28 days (discounting team members shared with other teams)

• Estimated focus factor: 70%

• Estimated velocity: 19 ideal days (derived from: 28 days * 0.7 focus factor)

• Actual velocity: 9.8 days (we finished the whole Sprint in half the expected time, clearly overestimated)

which gives us a focus factor of...

• Actual focus factor (to be used in the next Sprint): 35% (9.8 actual velocity / 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?