I recently had a "eureka" moment, or perhaps a "duh" moment working out some sightlines for the two walnut armchairs that I am making. ***please see the edit at the end of the post***
I've shot a short video explaining the details.
Hopefully the video clarifies the process enough, but if not, here is the brief synopsis. The sightline is based on a relationship between the amount of rake and splay that you want. If the rake and splay are equal, the sightline will always be 45 degrees. By using a straight edge with marks based on degrees from a single point, the incremental increase of distance is accounted for and we can use this scale for direct layout. If you are dying to know why this works, take a look at this drawing of the graphical process. (and perhaps visit this posting and it's second part for a refresher)
You'll notice that all of the views of the chair come together to make a small triangle. Well, by using the special rule based on degrees, we skip all the steps and simply make the triangle directly on the pattern. What enchanted me about this was the ease of finding the resultant angle by measuring the hypotenuse. Perhaps I'm just a total nerd, but this one made my day!
*** One of the comments that I got questioned whether the Bevel Boss scale was created by measuring from a single point for all of the angles. This method presupposes that it is, and it isn't! That's the bad news, the good news is that the angles below 30 degrees do converge nicely and since theses are the numbers usually used in chairs, the scale should work fine. .***