The reason I chose to have the lines mark the bottoms of rafters (and it is my choice, it's the way I do it, not the way it has to be done) is because it simplifies things for me when it comes to actually laying out the joint, assuming that other lines marked edges and not centers. If you choose to use your horizontal lines as center lines then your rafter lines have to be used as center lines too. I guess if you get right down to it, if any one of your primary lines mark a center then all have to mark a center, and if any mark an edge then all have to mark an edge, in order to keep joinery consistent and matching.

I thought I'd present that idea, assuming that it was normal to use the lines as center marks rather than my idea of using them for edges. It was presented as another possibility.

I think of it like this: When you are laying out studs for a stick framed building, do you make your marks on your plates for the centers of the studs? If so, you do it much different than anything I have ever learned. In stick framing, you mark the edges because it makes layout easier, and it also makes measurements conform to standard units. That's the exact same reason why I thought to do that with my daisy layout lines.

You may see logic in doing it otherwise, and I have not problem with that at all!

The rods to set up triangles is a great idea, thanks for that! I have found the string setup to be quite unsatisfactory because string tends to stretch, throwing off your accuracy.

I should talk a little bit about triangles. Triangles are my way of transferring things from the circles of the daisy wheel to the rectangles and lines of a building. I mentioned earlier that triangles can be used to mark rafter plumb cuts (and horizontal cuts) They can also be used to establish perfect right angles -I use the 3:4:5 side ration right triangle. If you make a triangle with a side relationship of 3 units for one side, 4 units for another, and 5 units for the last, then you will always have a perfect right triangle, which will allow you to lay out perfect right angles.
So with my fondness for triangles, I am very excited to have learned this trick as to how to use them! Thanks a whole lot!

setting the dividers to a multiple of the distance is quite handy, and I think you will actually be slightly more accurate that way (my opinion)

One thing I would recommend, If you are laying out a full scale floor plan don't use your dividers to step off the distance -use rods (which I am pretty sure is authentically medieval)You make a rod with pointed ends, set to a distance determined by the calipers (2 or 3 or 4 times the size of a line on my 1:24 scale drawing, for example) Then you just turn the rod end over end along a string that has been stretched out to make sure you're running straight. This way you can easily lay out a building on grass or dirt ground, or even a somewhat uneven surface.
For e personally I find that dividers are great for stepping off timbers. They work best when their sharpened steel points can set into a surface just a little bit, but not cut in (like in dirt) I don't like using them on concrete because it's hard on their ends.

Also... I REFUSE to get my tape measure out! Except for maybe here and there to test theories and relationships (ok, so a I use a tape measure to rough cut logs to length before they are squared, and I use an inch ruler and a bubble level to mark the size of timbers to be hewn)

and Ken, thanks for the info about the spread of the daisy wheel