Toivo, I couldn't make sense of your post but the last part. Please don't drink and type smile

I understand to layout on the floor(hopfully one that was poured pretty flat). I'm thinking that your "wonky" timber is above held up by timber ponies. I'm thinking that If you have the mortise drawn on the floor, you hang the plumb bob against the wonky timber until you find the corners and mark them? But if the face is curved, how to project square lines? In square rule, we were very precise with marking layout with a razor knife. How can you make markings that accurate on the floor? Boy I'm all confused. Thats why I need help to understand this.