Mike; Here is my old fashioned math. Your octagon is going to be about 90" as stated above. You are going to need 10' side to side to give you the 12" overlap you want. At 16" X 16", 7 tiles across, with a little sand between, will give you the 113.71 inches you need without cutting any tiles, for a total of 49 tiles. If you want to keep the 12" border all the way arond, you are going to have to do some cutting. Buy yourself a diamond cut off blade made for cutting stone or cement that fits your power hand saw ( HF ) and a good dust mask, safety glasses, and hat. Also take note of the wind direction. Being that all your angles should be the same, I would make up a jig to cut and hold the tiles. I would also buy a few extra tiles so you have the same dye lot and color, and to replace the ones you are going to break or cut wrong. Trust me here !! You can always return extras or make a little walk way to put down a mat to wipe your feet. What type of structure are you going to put up ?? Wood, metal, screened, or open ?? Jim S