AZ Jane

I am attempting to figure out the measurements for a Double Irish Chain quilt.

The plain center square, coffee cups and pots, measures 8 1/4" unfinished.
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I thought my squares should be 2.5" but it doesn't work out, center block measures 12". How do I figure out, what I should add to the center square and what size squares should I be cutting for the other block??
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Thanks in advance, before I have to go buy more fabric because I cut it all wrong!!!!!