I start by "aiming" the Pbox so the tip of the finial points to the approx. center of the wrist. Sometimes this is violated a bit if it doesn't look right or if the Pbox is on the large side.
A far as the length....going too far forward into the wrist baluster can cause abrupt contours w/ the finial. As the Pbox is being designed, this should be considered and also the areas adjacent to the Bplate should allow some fore and aft movement.
After achieving the final Pbox design on paper, an overlay of tracing paper that has the design penciled on is positioned on the stock as per the first photo...it's moved around starting w/ the finial pointing to the wrist center and if this doesn't look just right, the tracing is moved until it does look right.
When the best position is found, the tracing is taped to the stock and orienting prick marks are made through the tracing for locating the actual Pbox.
To start the inletting, the finial is formed to the stock contour using the prick marks and the center screw holds it in position. Usually a groove has to be cut for the hinge. Once the finial is inletted, the lid is next followed by the sideplates.
The main point of all this is that it must look "right".Some other builders probably use a different system that works equally as well....Fred