OK, so I am lazy. I over cut. I multiply the unfinished block by 1.8 and divide by 2, then square up. I cut on the diagonal, sew on the blocks then trim the whole block. If I have enough fabric, I will also cut the blocks for the setting triangles the size of the finished block, cut on the diagonal and sew. When I trim what is important is that I leave 1/4" between the point created (in your case) by the white fabric and the trimmed edge. That 1/4" is your seam allowance so the point of the white is exactly at the seam.