I don't think 9 is necesary, but trim the positions of the sensors based on the width of the line. I think 5 would be good:
Code:
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1 2| 3 |4 5
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1,5- perimeter, just to make sure your bot doesn't get lost.
2,4- if these hit the line, turn that way.
_ 3- Keep this on the line.
ps-the code was just to get a fixed-width font.