That seems like a lot of measuring and work to finally get to the HSTs. I prefer the Easy Angle ruler for HSTs. If you want 2.5" HSTs, you cut 2.5" strips and then cut the triangles down the strip with the Easy Angle ruler. Quick and easy and they go together well. I rarely need to trim anything after sewing.
The Easy Angle ruler was the best thing I ever learned about from Bonnie Hunter.