I started with the geometry of a bike that I knew I liked, then lengthened the head tube about 4mm, bringing the imaginary bars up and back a little. I entered the sloping top tube (6 degrees to match the lugs), and the offsets for where the tubes enter the lugs (how far one tube extends beyond the intersection with another). I also tweaked the water bottle locations.
I'll print this full scale so that I can line the tubes up against it. It'll let me quickly double check pieces and dimensions without TOO much measuring.