The angle depends on the number of pieces in the center of your star. If I understand you correctly, you are making a lone star with 7 strips in your strip set, but you will still have 8 diamonds in the center, like the quilt in the picture below. If so (8 diamonds), you would cut the same angle from your strip set no matter how many strips it contains.
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http://www.quiltingboard.com/picture...t-t180011.html
(thanks Luv2sew4fun!!!)