Starting with a 10" square and then making a couple of HST's with part of it, will never come out 12.5".
If you make each part of the 9 patch with 4 - 5" square, after you take off the 1/4" seams for a finished block, you will only have 12.5". By having to make HST's you are going to lose more size. Then you cut it twice, losing another inch. The final finished could only end up about 11 and that's if you don't do the HST's.
I haven't made the block, but since you will be doing the same thing to every block, and these are usually the only blocks in the pattern, it doesn't really matter what size you start with, they will all end up the same.
Having said that...The Hour Glass block is a 9 patch block, with a couple of HST's thrown in for fun. The same principles should apply to making a disappearing hour glass that you do for the D9P. Please let us see what you come up with. I always love seeing new stuff.