Adrian
Some contemplation indicates that if the pin is placed too far back then the workpiece will rotate in a direction that pushes the blade left. Similarly if the pin is too far forward than the workpiece will rotate in such a way as to push the blade to the right. A quick test on the saw confirms that this indeed happens: I can see the blade being bent in the predicted direction. I suppose this supplies guidance for adjusting the setup. But I'd rather not have to cut 3 circle test pieces before I can cut the actual circle I want. It seems like the correct position for the pin is on a line perpendicular to the lead line. But what part of the blade should it line up with?

Note that if the pin needs to be on a line perpendicular to the lead line then it's not clear how a device that makes use of the miter slot can work properly, unless it has a swivel adjustment built in somehow. (I've never seen anybody propose a design like that.) The only design I've seen for a circle cutter that accounts for blade lead is a T-shaped design that you can clamp on the saw at an angle.