If I'm reading this right, you are doing a HST with one side done as a QST.
I'd start with a 9" square and cut diagonally for the solid half
I'd cut a 9.5" square cut twice accoss the diagonal, and use one piece sewn to another for the other side of they HST.
I'd mark the middle of the solid triangle and line up the seam with that. Then I'd trim the whole thing down to 8.5", lining up the center point at 4.25" and keeping the seams at the point. This means you'll trim a bit off each side.
If you have some already done that are too big, as long as you line up the center and seams on the diagonal, you should not loose any points in trimming. That will only happen if you line up a side.
This is why I love using my Easy Angle rulers, I can cut a strip at 8.5 and then use them to cut the exact triangle I need (it eliminates the "dog ear" part so you can use the exact measurement).
Last edited by Macybaby; 03-07-2018 at 12:55 PM.