12 sided cutting

I want to cut a symetrical 12 sided shape that is about 9" at widest. Im using a miter set at 30 degrees. I set a stop, so each cut is the same I experimented (luckily) and I cannot figure out how to cut this shape out and get all sides even. Some sides are much to short, others much to long etc.
any clues on how to do this properly?
thank you
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