I see one step I make missing from what you do. After I cut in half, I press them open, align the 45* line on my ruler with the sewn seam, then trim.
For any size of triangle, I start with squares that are 1" larger than the finished size I want. In theory, I should be cutting 1/8" off. However, since I never mark either seam or cutting line, my seam allowances on the triangles tend to be on the smaller side and I tend to cut off a bit more than 1/8". My finished triangles are always exactly on, which drives my careful guild sisters crazy.