Mark,
Could be. I like the idea of a transit.
Check out this description of square rule from a 19th century carpenter.
Light and Heavy Timber Framing Made Easy by Fred T HodgsonWhen framing to lines, this is more or less the method we use.
By the way, I saved that entire book as a pdf, took it to a printer, and had it printed and spiral bound for about 35 bucks. It's public domain, so copyright laws don't apply. And it's a great read.