Cut 4 triangles out of a wide 2 by, at 45 degrees. Attach these with screws to the bottom side of a piece of plywood to form two V shaped saddles about 10 inches apart underneath the plywood, which will saddle solidly onto a corner of the post. Attach to the post with clamps or screws. Attach a strip of wood to the top of the plywood for a guide fence for the saw, to keep things square. You now have control of the variables most likely to give you trouble. Make your cuts, then chisel out the excess wood. Multiple cuts will ease the chore of chiselling.