From Vision to mission A mission statement should answer the question:
For a startup it is important that your mission statement is to the point, so that the audience of the business plan can figure out immediately what your venture sets out to do.
The mission statement should be short, but not shorther than that you answer the questions above.
Use verbes and adjectives that correspond to your competitive advantage, the industries key success factors and the preferred segments unique sales propositions. This will sharpen the business mission statement.
If you are a startup, it might be reasonable to go back and redefine the mission statement after making your strategy. For a more mature company it is semi-permanent as it is in the bylaws.