What percentage is needed?
Concept XII suggests discussion, vote, and substantial unanimity. Tradition I calls for unity. Does this mean you have to get everyone to agree?
With major decisions affecting the form and content of the group, you want pretty much everyone on board. After a group has opened up, you want to bring as many people as possible on board with major decisions, which often involves compromise. When you're starting a group, however, just find people who agree on what they want. Don't bring in too many people right at the start, as you'll end up compromising and forming just another middle-of-the-road group that no one particularly wants or needs.
Once you're up and running, there are two types of decision.
Further major decisions on form and content. These require two-thirds in favour of a proposal for change. That means that, without two-thirds, you stick with the status quo. This ensures continuity, stability, and the avoidance of contentious risk-taking.
With procedural decisions or decisions where the status quo is not available, a simple majority is enough, although more is desirable.
Procedural decisions are to do with how a particular job is done, e.g. how the timekeeper operates, what's in the script, what time the tea person has to turn up, etc.
Decisions where the status quo is not available are decisions for instance on whether to move to venue A or venue B, when the existing venue is closing, or when and where to hold a group conscience meeting. In both cases, there is no fallback position. 50% has to be enough.