My main strategy was to abandon all the exact measurements, and just trying to adjust everything so that it looks fine enough =) I wasn't that worried about the corners, as there will anyway be a 5" or 6" board installed vertically on each corner, so that the ends of siding boards will be hidden. So, if the siding doesn't exactly fit in the corners, that will be dealt away with those vertical corner boards. Or, that was what I thought =)

Looking at the first picture, you can see that the upper half of the north wall is slightly bulging outwards. The old log walls aren't standing upright in a straight line, but every wall makes a smooth S-shaped curve. I thought that I don't care that much if the walls aren't exactly right - as long as they can support the weight of the house, it will be fine. But still, only when I had half of the house covered with new siding, I realized that I'm going to have a problem when installing those finishing vertical boards to each corner. Since the siding reflects some of the S-shape of the actual log wall, I don't quite know how the straight vertical corner boards are going to sit on top of a curved wall. We will see =)

But, yeah, to avoid the problem I should have started with installing some upright boards on top of the log walls, so that the inner side of each board is more or less carved to meet the S-curve of the log wall, and the outer side remains straight. And then installing chipboard and siding boards on top of those upright boards would've resulted in straight corners, instead of curved corners.