I was browsing through the tool department at Sears the other day when I happened upon something called an "Ultra Rule". This is a flexible metal ruler perforated with holes just large enough to accomodate the tip of a mechanical pencil with a 0.5mm lead (which was thoughtfully included in the package). Because the ruler is stainless steel, a scratch awl could also be used without fear of damage. Either way, it should make it much easier to mark off precise distances. The holes along one edge are spaced every 1/32 inch, and along the other 1/20 inch.
I also found a metal protractor with similar holes spaced every 1/2 degree. Both tools are made by General, and can probably be found in other stores as well.