Finding missing dimensions.

To find missing dimensions on drawings or prints I found a free download "Screen Ruler" you can calibrate if even when you zoom in on small areas.
There is a PDF program called Nitro Pro which also has a measuring and mark up function. Allows you to calibrate it off known dimensions.
I have also been experimenting with Photoshops "Perspective Warp" function as well. Not used it for any GT40 tasks yet though.
I have also had good success by importing the scans into autocad or a similar cad software and then scaling according to the grid or a dimension.
Then you can measure the rest.