Explanatorium

In the end I used a thin plywood sheet to check the joint for squareness at the final fit for the junction between these two timbers. The cross marks on the ends of some ot the timbers are visible and I have labeled two of the ink lines on this picture.
After marking the end of the pillar again I hammered this joint apart one more time and made a few more trimming cuts to adjust it and then put it back together one more time.