I am a firm believer that things were done in many ways, and it sounds like the article which I am not familiar with states clearly that the ring of sill timbers were probably Hewn, Mortised and Framed in the yard close to the construction site. I would expect that at this point the seatings were all completed.
At the same time or previously the stone walls were raised ready for the ring of sill timbers. Once in place on top of the stone walls the timbers were probably wedged and levelled up and then thin stones and mortar filled any gaps, which would be normal in that case.
In our case we did not have stone walls but individual large flat stones placed so that they were under the load bearing points.
We also levelled up the sill timbers as best we could without a transit to work with, the main thing being that no wind in the timber placement was allowed. We then filled in the gaps with an historic mortar mixture which was 3 sand and 1 of slacked lime.
Whether the original timbers were seated and how much is not an issue because we were using freshly hewn timbers which were as I mentioned oversize, I think that I mentioned 3\4" roughly.
I hope this answers your question I have did the best I can to make it clear for everyone.
You did mention that you wondered what the level reference was well for me it would have probably have been no doubt 3 different places 1--the seatings on top of the sill, 2--the framing line on the side of the timber sill there from the work of the framers), or 3--the half lap mortises on the corners. Any one of these spots could have been used as a levelling reference point
Thanks again for the additional dialogue I am sure that many are following this conversation along
