Here is a practice block I made with scrap fabric. I did make the pocket. I thought it would be a cute orphan block for a donation quilt.
My block is close to 12.5. It is somewhere between 12.5 and 12 3/8. When I make the next one, I will cut the first four squares as a generous 4 7/8 and the second four squares as a generous 6 5/8.
I cant believe how much trouble I have with SIS. I wish I could get consistent results with triangles. My snowball method wastes fabric. When I use triangles, my center square always ends up too big; and I lose my 1/4 seam allowance when I trim it. On one BOM, I bet I made 4 versions just trying to keep that seam allowance.