I consider that a good solution. U-Boats had separate recognition manuals for warships and merchants, so if you want to do one only for merchants that's perfect.
If you are then using a mod for the periscope like mine, eliminating the automated distance measuring, you can go way faster with a printed recognition manual and a table for distances than with the ingame tools
EDIT: I forgot: Someone did already long ago a set of pictures with the silhouettes of all ships, you can download the pictures as they are already in the style of those WW2 recognition manuals.