[Mike Dodson]

Mike,

I use a measuring tool that is called proportional dividers. You simply check your reference with one end of the dividers and make your mark on your canvas or paper using the opposite. The center screw can be adjusted to what ever ratio you desire. Attached is a commercial set. I use a set that I made. I typically mark a vertical and horizontal line across the center of my reference and then do the same on my canvas. I then measure everything from the center of the two "cross-hairs" and transfer the measurements over. You just simply use the opposite end of the dividers that you are measuring with to make your mark on the canvas. The accuracy is dead on.