The math is actually pretty simple. In each district there is a whole number of delegates awarded to each candidate. The vote share is rounded to the nearest number of whole delegates. With 8 delegates the midpoint between 4/8 and 5/8 delegates is 9/16. That translates to 56.25% in a two person race, so any amount above that rounds up to 5/8. The 2008 Obama campaign was masterful in targeting CDs around the country where they were close to the point where one rounds up instead of down and consistently picked up extra delegates in districts.
However, in a big state, there's no way of telling whether an extra vote might make the difference with the statewide delegates. California has a whopping 158 statewide.
True, it's harder to move percentages in a large state. That's why Obama's team polled districts and targeted resources at delegates awarded by CD.
See PA for an excellant example of this.
