i think to get it turn out the size you want, you cut it 1/2" larger. then when you mark that 1/4" all the way around, it would give you 1/2" less as the finished size. then you would pick up the rest of the directions and go from there. i've done it 8" the way she did.
since measuring isn't an issue after the marking of the 1/4", size doesn't matter. the import thing , i found, was the layout on the cutting mat. you need to follow those directions carefully. once you do it one time, it's yours forever.