There's two approaches.
1) Just don't add points for each habit you don't complete, which
means you slowly rise in points even if you only complete one habit.
2) Subtract points for each habit you don't complete, which means you
need to complete at least 1/2 of your habits each day to move up in
points
#2 might seem more sensible, but I actually prefer #1. People already
feel guilty enough without losing points, and the punishment aspect
serves as a discouragement to check-in on unsuccesful days.
Hope that helps!