I think that I have got what I need from pictures. I will only learn more by actually giving it a go. My plan at this stage was to take levels of the highest points on all the stones where the post would sit. Then I would cut all the posts something like 3" overlength (whatever is enough to compensate for variation of the funkiest stone) but making sure that each post is exactly the same amount over length. I would then stand the posts (well bents actually) on top of the stones where they are to rest, plumb and level them. Set the divider the same width the posts are over length and scribe away. Pull it all down, get to work with chainsaw and gouge and then stand it all back up again. Step back, admire my handiwork or swear loudly. Either way I should learn plenty and be able to do it better next time.
Any major faults in my train of thought?