ImageJ (from http://rsb.info.nih.gov/ij/)
Working scanner (can be hard)
Set to Grayscale.
Set to 600 ppi.
Set to wtb points.
Set to Custom ~130,255.
Set Preferences -> Scan Material -> Positive.
Set Preferences -> More -> Scan Quality -> Quality.
Set Preferences -> More -> Interpolation -> Speed.
Go to Analyze -> Set Scale.
Over there change:
Distance in Pixels -> 600
Known Distance -> 25.4
Pixel Aspect Ratio -> 1
Unit of Length -> mm