OK, so you are making a 9patch as the alternate block instead of a four patch, right? Sorry to say, that will be almost impossible to measure accurately, since 10 inches (finished) will not divide evenly by 3..it comes out to 3.33 inches, for which there is no measurement on traditional quilting rulers. It would be even harder to subdivide that 3.33 inches to get a smaller multipatch square. I'd change the pattern and instead of making a 9patch, make a fourpatch as I described in my origonal message. hard to know what pattern you're working on without a picture can you send one, or a link to a picture?