Partway through the plotting. The lines radiate from the viewpoint (Brantwood) through each contour line wherever it crosses a ridge. Every fifth contour is highlighted in green to help keep track of the altitudes. I made a template to show how far above the horizon each contour's altitude would appear - this reduces with distance from Brantwood of course.
Once this was sorted out, the actual plotting went quite quickly. It was very satisfying to see the shapes of familiar fells appearing.