A 9 patch is 3 across, so 12" finished divided by 3 is 4" finished equals cut 4 1/2" allowing for seams. This is how you would go about figuring it out. A suggestion for you when trying to figure out a new block. Draw it on paper, color and number the various pieces and then cut them out. Remember to add in your 1/4" seam allowances (even if it means laying another bigger piece underneath). This will not only let you see what each piece requires, it will assist you in how to lay them out and figure the needed yardage but it will give you a cheat sheet for when you are assembling it. As you get a bit more experience, you will not have to do this, but, it is still often helpful to make a practice block of a new and tricky pattern to find the problem areas. Save those practice or orphan blocks and later you can stick them on the front of tote bags as gifts.